After reading the wiki about B.F Skinner and his work (here) its easy to see that most video games follow these ideas in psychology. The main idea is, if something good happens, a person will usually repeat this action, but if something bad happens, a person will usually avoid it. So for games this obviously applies to rewards such as power ups and health, and to conflicts such as enemies - unless the enemies are worth the reward that a player can get from playing them.  The reward might be health or an idea - or sometimes the reward is just being able to progress.

Just something to think about