Last week flying to see my mother for Christmas, I actually did feel bad -- not a jerk but bad -- for saying no. I was seated in my aisle (18C on a 777-200) when a family boarded. The young daughter took window seat and the mother took the middle. The father looked embarrassed and asked if I'd consider switching seats -- I had actually been about to ask him where his seat was because I was going to offer -- when he said he was in 16E. Boom, not just a middle seat but bulkhead so I'd have to find an overhead for my backpack.
Sorry, that wasn't going to happen for a 7 hour flight. I'd have swapped with him if he'd had an aisle (maybe even a window) even if it had been all the way in the back but center-middle bulkhead was just a no-go.