Forgive the brevity of this post, but I am quite exhausted and am having a hard time focusing right now. That being said, people game for all sorts of different reasons. Some people do it to escape the monotony of their daily lives, others do it simply for the social aspect, while others enjoy the mental challenge involved. There are plenty of answers out there, and most people probably have more than one, but there has to be one. Why else would you spend 4 hours every week, or couple of weeks, pretending to be someone you're not, doing things you can't really do, with the only interface being some polyhedrons and your imagination?
Personally, I game to escape and to vent. Gaming gives me a chance to take a break from my life, not that my life is so horrible that it needs it, but it is still nice to get away from it for a while and to venture into other lives, other perspectives, other worlds. I can be something I'm not. I'm a rather ungainly, somewhat socially awkward nerd in real life, but at the gaming table I could be an elven princess, or a samurai, or a hardened merc, or hell, a less socially awkward nerd. The possibilities are endless, and I enjoy the getting away.
By venting, I don't necessarily mean anger or aggression, though there are times when it can happen. Seriously though, video games are much better outlets for aggression, at least there you are actually pulling a trigger. No, I use games to help vent ideas and creativity. I craft worlds, put together stories. They don't have to be gone into in depth, nor do I have to do all the monotony that comes with trying to right a novel, but the idea still gets vented out. Character ideas, plot hooks, scene ideas, lines of dialogue, I can get them out, or at least enough out that the rest aren't driving me nuts. Not to say I'm some 'constantly inspired' muse hog, but anyone who has gone deep into the creative process (I hope) can attest that once you start, your brain will start bleeding ideas.
I can also use game to vent other emotions. Perhaps not rage, but idealism, loyalty. I can vent my need on a given day to be the person that everyone can depend on. Or my need to be the person who just leaves when things get rough. That is also escaping, but what is escaping if not jettisoning and venting your needs?
Aside from those, there are other reasons, the story, the mental challenge, the social aspect, but those are the two big ones. So what about you? What brings you back to the table every session? What has you there constantly wanting more?