Why won't guys go down on a girl?

I've ran into guys who've done it to me, but for a little, then guys who say it's their favorite thing, but I've never gotten the opportunity to do it, and they "like to do it", and guys who tell me they'll do it to me, but don't like it, so why don't guys like it!? is it gross?! Why won't you a guy go down on a girl?