Against a better player? A worse one? A jerk? What would make you refuse?
Accept a rematch?
the only time i would not accept a rematch is if the person is a jerk, but so far i have not had that experience!! 
on Live chess? i rarely accept them and never ask for them, inculding when i lose....
lol! well i have lost on time before and i was winning, so when they do ask for one i take it.
I always accepts the rematch as I have a little bit of trouble finding good adversaries. Sometimes though I am stuck with a less good player who had about the same rating as I 5 games ago but I feel as long as he wants to get trashed it is ok for me.
mvh Fredrik
Mostly I accept, but sometimes I regret it. For example I won a difficult game, put a lot of effort, then opponent wants rematch, I play and I lose because I am tired and play worse, than usual. Same, if somebody asks for repeated rematches, it seems me that sometimes they want you to get tired and lose. I do not think there is any obligation to accept, it should be considered as courtesy. Funny thing, I played one guy and he as black develops his pieces very lousy, so I just sacrifice bishop on f7 and give him checkmate in a few moves. He wants rematch, plays white a bit better, loses, then again rematch, I am white again, same lousy development, again Bxf7+, again mate in a few moves. I decided that this is enough and refused the 4th rematch.

When would you not accept a rematch?