summary: Games are so addictive because they provide a much more instant and gratifying sense of accomplishment, even when the times between accomplishments are boring as fuck, because the player will be geared toward making the accomplishment, and will block all thoughts of monotony out of their mind.

other point: Games need a sense of a "lottery"-like style where everything is randomized. say you have a rare drop at a location. a player will be there for hours, never getting off, because they know that the next drop could be the rare! compare it to the kalphite queen and dragon chainmail in RS, or to grind on leprechauns until you get steelies in ms. every monster you kill increases your chances of getting the drop on your next kill, so you stay hooked and keep playing until your girlfriend/wife/mom butchers you over being on the computer too long.
= variable ratio schedules.