I don't have an answer either. But it was kiind of funny that I was wondering the same this week. I was playing soccer and as always I playes too wild. So I fell on my knees. So the first questions of the others was: "Does it hurt?". Well, it did, but I said it was nothing to worry about and (forgetting about they're all being vanilla) I kind of enjoyed the feeling. I just loved the feeling of my knees burning. And it actually (weird me) turned me on.
Too bad my Master wasn't near...
I think trust is very important for me to enjoy pain. When there's no trust I don't even feel it or don't like it.