Originally Posted by MSPeconomist

Just say no. There's no need to explain or give a reason why you selected your seat. Too often people trying to change seats want the better seats rather than being willing to take worse seats in order to sit together. Moreover, in some cases they are seated apart because they refused to pay seat reservation fees or they deliberately picked one good seat hoping to get another good seat next to it rather than picking two less good seats that were adjacent.

If someone rudely demands that I switch seats, I will say no rudely in return and I will not feel at all guilty about this.