Was one on a LH fight YVR to FRA. I'd got to YVR early to get an exit row.
When I board some guy is in my seat so I ask him to move.
He gets into a strop because I was in the seat he would have asked for if he'd got to the airport early enough!
He then asks (more like tells) me to move and I refuse.
FA is now hovering and so involves himself and basically instructs the guy to either go to his assigned seat or deboard himself.
German authority won out and the guy moved but I swear the tears were flowing. I saw him later in a middle seat in the back row so no wonder he wanted to move!
Generally I am happy to move, if (a) asked politly, (b) for valid reasons such as allowing a couple/friends to sit together (c) only to a seat of equal or greater value!
Btw if in a row of three and a couple are next to me I insist that the person in the middle disturbes their partner when they want to get out too!