Here's another perspective on engagement: how to make games more addictive (or compulsive?) by creating "smarter game loops" with fewer exit points. The example discussed is "Game Dev Story," a management simulation for a game development studio.
In this talk, Csikzentmihalyi goes further than describing flow as an important experience for intrinsic motivation. Is happiness a state of flow too? It is an interesting perspective though I'm sure it's not the only perspective.