The other answers are excellent, and correct. I wanted to share a few extra ideas because you specifically said:

Do note that I am very shy and submissive in public.

I also am a shy person that's, for various reasons, done a lot of travelling on my own. What I always say to myself is:

You'll never see any of these people ever again.

And that gives me (in my mind) permission to be someone else, to tell people no, to argue that I'm due the seat I paid for, or to sing karaoke, or whatever. Particularly if they're not asking nicely. Any embarrassment I feel is going to disappear at the exit of the airport, at the latest. Travelling alone is a great chance to try things you normally wouldn't do -- and, who knows, you may feel comfortable doing them when you're back home as well. Everyone else is right, it's completely your right to refuse, but if that's really against your nature just pretend to be someone else for the flight. Don't go overboard, but you can be someone that's pleasant but firm and fair -- and who doesn't worry about it,

That's one side of it, but from experience somedays you just can't, on those days I use avoidance tactics that you can employ too. Put headphones in when you sit down (there doesn't have anything playing), close your eyes and relax. Only the rudest of the rude people will target you to ask to change seats in that situation. And you'll avoid any small talk with the people next to you if you're not feeling up to it.

But, above all, however you do it, do what everyone else said, and just say no.

Unless it is a safety related reason, it will be very hard to change your seat without you willing to do so, that includes cabin crew.

So, as mentioned in the other answer by @CGCampbell, just politely say no. You might get frowned upon but who cares! it is your seat and it is totally your right to be stuck with it.

Something worth mentioning here, airlines in general try their best to put people in their preferable seats, but it is almost an impossible job to do, so ground agents try to spare of that headache to cabin crew, who also will try to spare some of that headache to passengers, so it is not abnormal to see cabin crew "hiding" during boarding to avoid the extra headache and embarrassment and let passengers solve things. If that ever happens to you and you felt uncomfortable by requests, just inform cabin crew. If they were available most likely passengers will not bother fellow passengers and will try to ask cabin crew instead. Cabin crew are trained on how to turn down requests with a big smile.

Regarding punching people who ask for your seat in the face, no need to do that. Actually you will be giving them a big favor if you got physical or rude. Crew can then offload you according to the international "unruly passengers" regulations and then take your seat. So just say no and enjoy the ride.

One last thing, as a cabin crew I know of a few ways to make people willingly give up their seats. Don't fall for that. One of these techniques is asking a passenger with a voice that passengers around can hear and then saying something like "Sir, are you interested in switching your seat so that family can stay together", this will be hard to refuse, since refusing publicly will make you look like an evil. Do not fall for that, just refuse with a smarter answer, something like "I would love to, but due to my health reasons I specifically requested this seat".

As a young person, you should never, ever, swap seats with anyone.

Here's a thing for you to say:

"I am not allowed to do that. Please bring one of the cabin crew. I've pressed the button to call a cabin crew..."

Whatever the joker says back, just keep repeating, "I better speak to a crew member, here I'll press the button..."

Please note Vedant, you say

"I personally feel very rude by involving the flight attendant..."

Nonsense!

It is imperative that you involve the flight attendants. They are there to help you.

If the person beside you is acting weird, call for a crew member. If the person beside you is coughing all over you, call for a crew member. If some fool is talking to you about seating positions, call for a crew member.

This is imperative for younger fliers.

Sure, some adults have the aplomb to handle such situations on their own. But anyone who does not, and any young person must call for a crew member. Don't hesitate! They are there to solve any crazy problems other passengers are causing you.

"I'm sorry but I'd rather not trade seats."

If they insist;

"No, I am not going to be changing seats, I'm sorry."

Here's the economist's solution: Offer a price at which you are willing to give up the seat.

If he refuses the offer, then too bad for him. If he accepts the offer, then both of you are better off---for the price bargained, he gets the seat and you willingly give up your seat.

If you are very shy and have a lot of difficulty dealing with people directly, a very simple strategy is to lean back in your seat with your eyes closed and wearing headphones or earplugs until after takeoff. Most people will not disturb you while you're like this.

I realize this isn't a direct answer to the question "how do I tell them no", but it might help you avoid people asking you in the first place.

I totally feel you. I had this happen to me for a seat that I had paid extra just to be seated there. I politely informed them that I had specifically requested this seat and would not change my mind. The person seated did not bother to move and seemed indecisive for a little while and I just waited for him to come to terms with what was happening. Finally the person moved to their respective seat and I could sit in the one that I had paid for.

While I was standing, I was thinking about my options in-case if the person decides to sit there and not move, one of which was to inform the flight crew. Just like you described in the question, I also feel really bad going to a higher authority to get my problems solved, but in certain situations that might be one of the only ways to resolve the situation. I would suggest that you try holding onto the seat and being firm with your answer as you will never be seeing the person who you are removing from your seat. But if nothing works out, call the cabin crew and make up some health BS.

Another note that hasn't yet been mentioned is redirecting the person.

"I'm sorry you were not assigned the seat you needed. I cannot help you, but perhaps one of the attendants can. Shall I press the call button so they can help you?"

While the base action is the same as other suggestions - saying no and calling the attendant if pressured - the wording is designed to give them no way to argue with you, and immediately tells them what will happen if they continue to pressure you.

If they pressure you, or ask why you can't help them, simply move forward with the only thing you can help them with:

"Again, I can't help you, but I'm pressing the call button and someone will be here shortly who may be able to help you. I hope you are able to have this situation resolved."

Don't give them even an inch. Don't give them any information they don't need to know - and the only thing they need to know is that you cannot help them resolve their problem. Don't suggest you have a medical issue. Don't suggest you have a preference. Don't give them any opening which would allow them to argue with you that their "need" might be greater than yours.

Just say no, call the attendant if pressured, and ignore them.

I think what you describe happens quite often in parts of the world where the pecking order is based on age, so teenagers rank slightly better than animals, but only slightly. Older people try to take undue advantage of their position in the pecking order.

I have been in the same situation as you. I found that the best solution in such a situation is to say that you have an injury or problem, say problem with your back or your foot and so you won't be able to travel in the alternative seat being offered. This worked most of the time. Sometimes the person trying to take advantage of you might be somewhat obnoxious and they may persist. They may do one of the following:

1. Ask for further details of your injury or problem.
2. Claim they have a bigger problem than you have and you are young anyway.
3. Mutter something about inconsiderate teenagers or perhaps something rude, or glare at you in a nasty way.

For case 1, say "I have a very good doctor to treat me so please don't worry about it".

For case 2, ask them to switch seats with someone else who is healthy, as you are in the same condition as Mr. Obnoxious.

For case 3, ignore them. They did not succeed in taking advantage of you. They lost. You won. Do not be intimidated by them.

Try not to give off vibes of diffidence and shyness. Obnoxious people seem to sense that like a crocodile senses an injured prey in the water. Observe your body language and change it as needed.

Since I prefer window seats, this does not happen to me often but the few times it has happened, it is because it is a parent (usually father) who wishes to sit with their family in the other 2 seats.

As a parent myself, I can understand the desire to want to sit with your family, especially when the family consists of multiple young children. A grumpy young child can make the flight uncomfortable for everyone and if the parent thinks they can help keep the child calm, it might actually make the flight more pleasant for all.

However, if you need to sit in the seat you reserved in advance, then it is your seat. I should have no right to force you to move, and I personally, would not be upset by someone refusing to change seats because I asked. However, if the issue is only the need for the aisle and not a specific seat you might want to check if the passenger has an aisle seat.

A simple response might be

I had a condition that requires me to sit in an aisle. If you have an aisle seat in a different row, I would happily switch with you.

Then let the person respond. If they have an aisle, then switch and you don't lose anything. If they don't, then (hopefully) they will move on. As a last resort, the flight crew is your friend here. If the person is bothering you, they will support you.

It happened to me recently. I had checked in online and reserved the seats I wanted. A guy came to me and asked to change seats so he could sit next to his wife. I told him that my 3 kids were just across the aisle and that I wanted to be next to them. He then tried to convince me that the other seat had more leg room! I just said that I didn't want to change. He went off but the wife fumed the whole way. Her problem - not mine.

You reserve your seat, you get to sit in it - no matter what. The cabin crew want a fast turn-around so they back you up if you have the boarding card.

I would look at this way:

1. To begin with, the other person does not know or understand your need to sit in the aisle. If you tell them that you have special needs for that, or that you payed extra for it, they should be able to understand this.
2. If you tell them that you need to or payed extra for sitting in the aisle, then that person would be selfish to try to deny you this. As such there is no reason to be shy about answering no, you have every right to do so.

Since your problem is that you lose your seat because of shyness, you'll have to overcome that problem. Not the being shy, but the losing your seat.

The next time it happens, rule number 1: This is YOUR seat. YOU keep YOUR seat. That's the unavoidable outcome. You may become totally embarrassed because you have to say no to people or worse, which is unfortunate, but the whole situation will end up with YOU sitting in YOUR seat.

So when you are asked to change seats, since the outcome is unavoidable it can't be helped that you are less diplomatic and polite than you wish. So the answer to any request is: "This is MY seat and you are not getting it.". That's what you say. It my come as a bit of a shock to you and the person asking you, but its the truth. Fortunate for you, less fortunate for the others. It will come as a surprise to the person asking, so most likely they won't have any comeback.

If they continue arguing, your answer this time is "This is MY seat and you are not getting it.". Oh, that's the same answer. Well, it had to be, because of rule number 1: This is YOUR seat, and YOU keep it.

If you need to answer again, you may add: "This is MY seat and you are not getting it. We both know that. If you don't leave my alone I'll have to call a flight attended. "

A flight attendant will be only too happy to help you. Taking care of an unruly passenger is something that puts a bit of excitement into their life.

And I'll just repeat the number 1 rule: It's YOUR seat and YOU keep it.

You have a seat that presumably you paid for. You are not under any obligation to switch seats with another passenger, and you do not owe them any explanation. If someone asks, simply say, "No, sorry". Then turn away and direct your attention to something else, open a book or start using an electronic gadget or something, to indicate that the conversation is over. Do not continue looking at them and wait for an acknowledgement from them, because that just invites them to press the matter.

If you say no and the person pushes, I'd give the briefest of explanation. If there's some specific reason why you prefer this seat, say so. "No, sorry, but I like a seat near the middle of the plane" or whatever. Or just, "No, I like this seat." Don't yell and don't get into an argument. If the person continues to be insistent, say, "If you need help finding your seat, I'd be happy to call the flight attendant for you" and indicate the call button. If he still pushes, really do call the flight attendant.

Unless the other person has some good reason for wanting to change seats -- if he has a medical problem and for some reason your seat would be better or something -- I doubt the stewardess will tell you that you must give up your seat.

As someone else noted, I'm surprised that you say this issue comes up a lot. Maybe it's different in India, but here in the U.S., I think once in my life I've had someone ask me to change seats, and that was because a family had been split up.

I'll provide my take on this:

As much as I agree you can every right to keep your seat, remember that other travelers can be stressed out as well. Stressed people can do crazy stuff that is not in their regular nature. They might want to sit with their families or children and do not realize why you traveling alone cannot be bothered to change. (In all honesty, I've flown a bit and never considered this. I thought claustrophobic people preferred window seats). Their concern for their kid may be as bad as your claustrophobia. Saying no in this case, while you have your right to, might make the situation worse.

• They may escalate it to flight attendant and get more people involved, "causing a scene". This also would suggest they are not prepared to give in easily. If it can be solved at earlier stage ("Do you have another aisle seat for me?") it may work better. No toilet rows? For smaller aircraft (A320/B737) try "I have a short connection and would really like an aisle seat at the front of the plane".
• They might come over to your seat for the rest of the flight with a legitimate cause if they have kids that need to be attended to for instance. I might am not familiar with claustrophobia, but I don't think this sort of stuff would help if you are shy. If they are unfriendly with a legitimate cause, I don't think it will improve. My parents declined a seat change some time ago, only to have that person annoyed and bother them the rest of the flight.
• Test around what works best. For instance, use the gate agents to your advantage and ask how full the flight is. If it's not entirely full, try to board a little later. You will still have legitimate access to your allocated aisle seat, but you can also easily see if there are other free seats where you might even get two seats for yourself. Clear possibilities are easier to see than when passengers are crowded up in the aisle. Also opens the possibilities to go communicate via the flight attendant looking that everybody gets settled rather than the passenger occupying, who is normally calmer and more understanding of your cause.

If I would have somebody come up to me at a window or aisle seat and asked for a change and would softly hint at that they were uncomfortable, I would happily oblige to switch if it saves them from feeling shit the rest of the flight. As much as I realize and understand your cause (and your preferred response), perhaps consider picking your fight wisely.

I usually say "I will swap seats if requested by the cabin crew. Here, let me call one for you." And this is the truth, even when I move myself to another seat I always ask the crew whether it's OK.

I am normally happy to change seats as long as the seat is of equivalent comfort. It is just not that big of deal for me. I have changed when the seat was not as good just because I felt like being generous that day.

However, I was recently asked to change seats from an aisle to a center seat. A man wanted me to change so he could sit with his wife. I wanted to use my laptop and the extra space on the aisle was important to me. I simply smiled and very politely declined. Everyone around me knew I declined. Later he asked the lady sitting in the window to change with his wife. She accepted. Many of the other passengers heaped praise on her for changing seats; This was obvious a back handed way of shaming me for not changing seats. I dealt with it by joining in on the over the top praise.

Another time I was asked to change seats for a lesser seat on international flight. The gentleman knew it was not as good of a seat. He handed me his card and said I should contact him if I needed any assistance while visiting his country. He worked in the office of the President. I never needed to call him, but if I had, it would have been well worth changing seats.

Overall it is a no-brainer to switch if the destination seat is better.

I don't understand why so much animosity against people asking to change in some answers and comments.

I agree that asking to change for a better seat and sending the OP near the toilet is bad but that's not the most common situation.

I once asked someone to switch with me, to be near my family. No, this was not a trick and I didn't book my travel late. Nor I thought that doing it loud would get me some advantage.

Sometimes places are just assigned and/or you can't pick them. It will be hard to keep a group together.

I was careful enough to ask a person that was sitting in the same position as I was, near me, and apparently without family.

If the person answered that she didn't want to or couldn't change I would just accept it. I would love going with family, but its the person's place. I think most people respect that.

All this to conclude: Don't be afraid to just politely refuse as others already answered. You can also give an explanation. That usually helps others understand. Not that you have to do it, but it will give you a feeling of assurance and the other will also accept it better.

From a moral point of view, their request is identical to a stranger asking you for $20. They have the right to ask, but you have absolutely no moral or legal obligation to give. In some situations it will be nice to help them out, in others it will be stupid. But in no situation is it required of you to help them.

The only things you cannot/should not do:

• Curse at the person(e.g. say "F**k off")
• Get physical (push the person, hit the person).

If the other person does any of these 2 things the single thing you must do is shout "help" repeatedly until either someone intervenes or they move away.

To decline their request, absolutely everything except the above is perfectly acceptable. But I strongly advise against giving a reason, because that can lead to them trying to argue, which would probably be uncomfortable for you. Saying "No." is sufficient. So is shaking your head. So is ignoring them. Or holding out your hand with your palm facing them. At worst people will think that's weird. Remember, their request is the same as asking you for $20.

On a related note I also suggest you practice in front of a mirror two everyday responses:

• A friendly "No thank you."
• An assertive "No."

Just can give an explanation in addition to making clear that you are not going to give up your seat so as to make your refusal sound a lot more polite. E.g. I usually say that I need to visit the toilet quite frequently when I make clear that I'm not going to give up my aisle seat. In your specific case you could say that you need to get up to stretch your legs quite frequently which is going to bother other passengers a lot (e.g. after food is served) unless you have the aisle seat.

Just compare the following two sentences:

"No, I'm not going to give up my seat".

"Sorry, I have chosen this aisle seat as I have to get out of my seat quite frequently and I also don't want to sit in the back of the plane".