Looking back, mountain biking, particularly XC, is sadism at it's finest. During the race you feel your spirit breaking, you want to cry and give up yet you keep motivating yourself, "I want to finish this, I want to finish this," and you do. You don't win, but you did finish where many others pulled out. You're body is aching, and you feel like collapsing. At the same time, once you cross that finish line, you're like, "I actually did it," and you look at the expressions of those who didn't finish it and you feel a mix of joy and pain. Days after that, after you've recovered and gotten back in the saddle, you think, "I can't wait for the next race."
It's borderline stupid but it's addictive. That feeling when you're atop the hill and you know you pedalled and struggled your way up. You didn't take some big 4x4 and wreck the environment to get the view. You broke a sweat, you swore, you struggled, and when you make, it's that much sweeter. Pure, sweet hell. And you want to do it again.
Events like Downhill, Dirtjump, Freeride, etc take lots of guts, a pair of titanium balls and skill. The adrenaline rush is amazing. But it's not the same as the strange, addictive sadism of Cross Country riding.
On those crazy climbs, your legs are burning, your lungs are burning, your mouth is dry, you can feel your lips cracking & your heart is racing at a thousand miles an hour. As you approach the top, it just gets harder and you seem to be travelling at an increasingly slower pace. Once you've hit the top, however, that's when the rolling hills start and you look forward to that next climb.
It's that feeling of "I can do it" and the sense of triumph when you reach the top that makes XC riding so addictive.