In MMOs it's easy: go look at the psychology of rewards. Psychologists found that if you rig up an experiment where you take rats and give them a button where they get food when they press it they'll more or less do the sensible thing: when they're hungry they press it and then eat.<p>However, if you rig it up so that if they press it they some times do NOT get food they'll immediately press the damn button until their tiny paws fall off. (No joke.) Even though they could do the same thing (press it until they are no longer hungry), the inconsistent rewards makes them freak out. It's the same thing with MMOs--that's why MMOs have loot that you have a better chance of winning the lottery than getting: because it makes people play 24/7 until they die of exhaustion.<p>As for myself... i prefer to play games that aren't quite as blatantly copied from psychological experiments. Once you buy into the internal logic of the MMO you're done for until you crash and/or quit.