I just got my new computer system online, and reloaded Dragon Age. I ended up playing it all evening, in two-hour increments. It is pretty amazing how two hours can go by so quickly while only seeming to be about 25-30 minutes.
MMOs (multiplayer games) are even worse, if you are doing dungeons or raiding, since you are playing with anywhere from 3-50 other people, all in coordinated efforts, and you can't just leave abruptly in the middle. Some raids are also timed. It's one of those things where the group and the circumstance reinforces complete immersion in the task at hand, to the detriment of whatever is going on around you. At least single-player games, you can typically save and pause in the middle no matter where you are. With the raids, you are pretty much tied to the game until the raid is over, and if you bail, you're going to negatively impact everyone else you were playing with. It's very easy to lose track of real life if you're a committed raider.