Aside from loot drops and achievements, I think there is a real element of compulsion for some people when it comes to certain games, particularly competitive online games.
Back when I was in high school/college and competitive FPSs were really starting to take off (Tribes, CS, etc.), there were times when I pretty much sacrificed my social life to sit in front of the computer for just one more game. There was underlyingly a sense of not really enjoying it, but I couldn't resist just playing one more round. It consumed my life and after I would spend 8 hours playing and felt like crap, I would tell myself no more, but the next day I'd be back at it. It really resembled what I imagine a gambling addiction to be like, but thankfully I wasn't losing money, just time and health.
I was finally able to stop after traveling abroad for several months, and when I came back I just deleted everything from my computer and haven't touched an online game since.