Here's a snippet from the book:
> Many simple things can be made complex when you dig into them, but having fun is something so fundamental that surely we can find a more basic concept?
> I found my answer in reading about how the brain works. Based on my reading, the human brain is mostly a voracious consumer of patterns, a soft pudgy gray Pac-Man of concepts. Games are just exceptionally tasty patterns to eat up.
I do think that Uplink has some super tasty patterns in there, which is exactly what the author is talking about when he's talking about the feeling of noticing how much better you've gotten when you start the game over. Of course, fun isn't a binary classification, and there's definitely a bunch games that are very heavy on the 'juice-factor' and other such things in an attempt to bump up the fun.
[0]: https://archive.org/details/theoryoffunforgamedesign2ndediti...