Coming up on an ENWorld Gameday tomorrow, I find that this is an excellent question.
Main reason? Sheer love of the game - the number-crunching, and the coming up with neat concepts, and cool stories. Even if I never gamed, I would still buy new material and peruse books, comb the web, etc.
Second reason? Having fun with friends. I enjoy the socializing, the cutting up, the insults, the jokes, the sharing of stories and letting off steam. Even without the gaming, these are still the people I go to movies, organize day-trips, celebrate holidays, etc. with.
Third Reason? Because it's an excellent social outlet through which to find people with whom I have things in common. Just like a stereotyped sports jock would find a better home in a sports bar than an sewing circle, When I'm socializing or partying with fellow gamers, I don't have to expalin myself. We all know most of the same odd quotes, terms and culture. If I say to a gamer
"critical-fumbled on that pool shot," or
"I'm going to my family reunion and then I've got a Forgotten Realms Game after that," he or she doesn't give me an odd look.