[sph]

In simple, reductive terms:

Fun, among other things = remaining in the tight channel of flow, where your skills get challenged without ever reaching a point of frustration. Too little challenge = boredom.

Skills improve as they get challenged, i.e. when our prediction and pattern matching system receive enough feedback to improve upon our previous actions to get a more optimal outcome.

So, fun is (among other things) getting better at doing something, and as we get better, what was once a challenge turns easier, so a fun game needs to have a well-tuned difficulty progression to keep in pace with your improving skills.