There's nothing I like more than finding out what someone's sexual fantasies are and doing my damndest to fulfill them. To learn that a guy's always wanted to be tied up, or never had a proper blowjob, or always wanted a girl to do him up the ass... I love that I'm being trusted with his fantasy and I really love when I can make it come true.
In many ways, probably more deeply than bondage or pain or anything, that's my ultimate fantasy--to give someone exactly what he's always wanted. I'd make some snide comment about being a one-woman Make-A-Wish Foundation, but really, it's not pity or even generosity, it just makes me hot to see a guy getting something he's wanted for years. Maybe it's a submissive thing, maybe it's an insecure "if it's his ultimate fantasy I know he won't hate it!" thing, but more than anything it's a goddamn, the look on his face thing.
(It's also a teensy bit of a superiority thing, because I can do things that women who aren't as open-minded or determined to please won't, and I get to hear the guy talk about how none of the other girls ever did that for him.)
It's sure as hell not the only reason I'm kinky, but one of the benefits is that kinky men know exactly what they want. Vanilla guys who just want "really good sex" with a "really hot girl"... I can try my best but it's not the same as "I want you to force me to lick your toes" where I know I can do it just the way he likes. Guys who won't admit any fantasies at all are horribly frustrating.
It's an honor to know someone's fantasies and a huge goddamn thrill to make them happen.