Raph Koster has a good book, "A Theory of Fun for Game Design", which I think does a pretty good job as an easy read intro to the problem. My main takeaway fro

Raph Koster has a good book, "A Theory of Fun for Game Design", which I think does a pretty good job as an easy read intro to the problem. My main takeaway from it, if I had to boil it down to one point, is your brain finds fun in mastering something (beating a challenge), but not so much afterwards (boring/repetitive) or if it's too difficult (frustrating). The problem is, where that line is for any given challenge can be different for everyone. You can also get more mileage out of something if an aspect of the challenge changes (people will play chess longer than tic tac toe for example, even though the game space is the same each time).