Gamers Anonymous: What makes some games so addictive?

How is it that certain games can become addictive, while others seem to simply run their course before tapering off into that void of content that we feel has been satisfactorily exhausted? There has to be some key factor(s) in the distinction between addictive and non-addictive videogames (but still fun ones, because who really wants to play a game that isn't?

GamerNode's Eddie Inzauto got to thinking about this while playing Professor Layton and the Curious Village, which he feels is a solid game with nothing revolutionary or "must-have" about it - but that is still enjoyable... and addictive. Now why does a simple game full of brain-exercises compel us to keep playing fervently for hours?