If someone is already in my seat when I get there I think one of two things:
1. They are rude.
2. They are too stupid to figure out what seat they should be in.

I hate moving seats and can only remember doing it twice, once to let a co-worker on crutches have seat 3B on a RJ so he would have more room (I offered, he didn't ask), and another time I don't know why I moved, but I went back three rows, but still in the aisle.

One time a guy was in my seat in first and wanted to fight me for it! Literally, he asked me if I wanted to fight him for it - and he was big, like 6'4 300 lbs big. I told him I didn't want to fight him, I just wanted to sit in my seat, he reluctantly moved back to his window seat about four rows back - I sat down with a big sigh of relief.