I was once sitting in F next to a woman who asked if I would mind switching seats with her older teenage daughter, who was sitting in the aisle seat across and one behind us. Of course, it wasn't an issue going from one aisle seat to another. But the teenage daughter said something like, "No, that's okay, I'm fine here." The mother sweetly pointed out that she was nice enough to pay an extra couple hundred for her daughter to sit in F so that they could be next to each other during the trip, and this was all the respect the daughter gave her. The daughter then sheepishly changed seats with me. Maybe you had to be there, but it was pretty funny.
Oddly enough, a couple weeks later, it happened again with a young man who declined sitting next to his mother by changing seats with me in F. His mother was a lovely woman and turned out to be a good seatmate - short conversation followed by little more than polite contact during the flight.