Deriving "fun" from a game is not just a straight forward pursuit of happiness. In certain cases, frustration may be necessary for the bigger picture. Games are really about competing with and winning against yourself. Even if you are pitted against an opponent, you still need to elevate your abilities in order to win. The internal battle is still there. You have to work within the game to achieve this growth, and when you finally attain it, there is a sense of pride and satisfaction. I think that's where the fun is derived from. So for me, truly "fun" games are ones created with hard challenges that can give you that sense of accomplishment in the end.