Enjoyment is not a requirement of game systems in and of themselves, but is a necessary condition nevertheless for voluntary player engagement. Why this is so could be better explained through psychology. The neurotransmitter dopamine is predominantly responsible for decision making and pursuing pleasure. The interesting thing is that dopamine is not as dependent on the reward, as on the anticipation and prediction of receiving a reward. Our brains are set up to exploit conditions where more rewards (food for example) could be obtained. If one is able to successfully predict where the next meal will come from, they would have a better chance at survival. Thus, the brain aims to understand why it got that unexpected reward. That is why games like chess are so engaging; A victory is like finding food, and the player then works backward to figure out what they did that made them win. However, since the number of ways of winning in chess is near infinite, there is always some new path to understand, and thus, a constant stream of dopamine.
This psychological understanding of our brains presents an interesting consideration: uncertainty is enjoyable only when it influences outcome. By outcome, I do not mean the binary “Win” or “Lose” condition, but a broader, player defined state which she deems as being important based on her own inferences. This could be a crucial part of the story in Dungeons and Dragons or the condition where the player dies in Counter Strike while her team is still in play. Uncertainty will keep the players coming back to understand how things work in the game and what they can do to maximize their rewards.