This is one of those air travel pet peeves. You have carefully chosen your seat and when you enter the plane someone is already sitting there or soon requests a seat swap with you.
And the way these usually go, the seat that they would like you to swap to is not comparable to the one that you have already chosen.
You can access this New York Times article here of which below is an excerpt:
Turf wars over the limited real estate in a plane cabin, from the overhead bins to the armrests, have become more acute in recent years. And with airlines packing planes tighter and charging more for exit rows, for seats further up in the economy cabin or for seat selection at the time of booking, requests — or demands — to swap seats have taken on a new tenor.
“People know not all seats in the cabin are created equally,” said John Thomas, head of the global aviation practice for L.E.K. Consulting.
Carriers increasingly divide the cabin into smaller segments, more like the way seating in a theater or a sporting arena is priced. Industry experts say this gives customers what they want.
I was once called a d*ck by another passenger when I refused to switch my seat when flying on Virgin America domestic first from San Francisco to Los Angeles.
On one of my British Airways flight, the other passenger would have preferred my 1A seat that he had supposedly reserved but was moved from close to the departure. Refused.
If someone is sitting in my seat AND the offered seat is not in higher cabin, I always flatly refuse. By taking someone’s seat first and requesting swap when the passenger shows up, is just plain wrong and inconsiderate.
On the other hand, if someone nicely asks to swap a seat to a comparable one, I usually don’t have a problem with these requests.