Well...I always tell myself when I initially sign up for the race/event it's for "fun", but when I do go out there, it's to do the best I can--especially if it means to win!
I haven't done any bike races--yet, but I've done a lot of running races and a few marathons, I think it's the adrenalin--rush that I get when I'm almost towards the finish where I do a "kick", mind you, where THAT came from, I have no idea. It's always good training anyway, I think, and I've got to admit, lots of fun to run/race with a group of friends!
Fubar5, the pukin' stuff should be categorized under the "fun" category--definitely!
You can get a natural high and have with no side effects (or bad consequences for that matter)--what more can you ask for?