It's the seat holder that gets to decide what is "reasonable", not the requestor. Frankly, someone poaching the seat before I get there gets dismissed out of hand.

However, even a request to swap that may seem "reasonable" to the person asking (wants to sit with their spouse for example, usually a want rather than a need) may not be reasonable to the rightful owner of the seat who has any number of reasons to deny it and whose definition of reasonable the whole swap rests on. Even if by any, normal social standard the rightful owner has no good reason to say no, they still retain the right to and nobody else's opinion matters. They needn't offer any excuse. They have the BP for the seat in question. End of story.

I say no on principle, even if the swap appears equal because I have been flying along time and have ended up on the short end of the stick too many times. I select and pay in advance, sometimes paying a bit more. If I am seated, I simply cannot be arsed to move to unite a couple. I may move for someone with a clear need such as young kids, the elderly, or someone handicapped, but even then only if the FA asks and I don't end up in a worse situation on a long flight. It's not my job to accommodate anyone or solve anyone's problem.

Common courtesy is taking the seat you are issued, sitting in it, and not bothering anyone else.