Requests to change seats should be made to the flight attendant, not to the passenger. In literally hundreds of flights, I have been asked only twice to change seats, once by the flight attendant to allow a very elderly and frail woman to sit next to a member of her family, who was seated next to me. Of course, I agreed. The other time, a woman in her twenties, some 30 years younger than I, travelling alone, asked me to change my aisle seat for her center seat, and I said "Do I look stupid? Certainly not!" OK, this wasn't tactful, but this was a time not to be tactful.
If the request does not come from a flight attendant, just say
Talk to the flight attendant if you don't like your seat.
If the request comes from a flight attendant, consider the reasons and either say yes, of course (as I did for the old, frail lady) or no, sorry, I'd much rather not. Do not engage with a pushy passenger.
Some readers will correctly object that flight attendants are too harried on today's crowded flights to do much seat-rearranging. Precisely! But they will make time to deal with an urgent request involving the very old, the disabled, and young children. All others should just settle in and endure.