The Multi Billion Dollar Mind Trick: Manipulate People by Simulating Success
Computer games are based on psychology, they try to make people think they accomplished something with as little effort as possible. Millions of people are addicted to it.
I think there's got to be a perceived effort and learning curve involved.
The tricky part is to get people coming back. That involves increasing the difficulty (but not too much), so the user feels a sense of accomplishment.
Throw in elements of randomness and, voila, you have an addictive game.
There are studies that go into more depth than this, but that's the basic outline.