Fun theory is the field of knowledge that deals in questions such as "How much fun is there in the universe?", "Will we ever run out of fun?", "Are we having fun yet?" and "Could we be having more fun?"
Fun Theory is serious business. The prospect of endless boredom is routinely fielded by conservatives as a knockdown argument against research on lifespan extension, against cryonics, against all transhumanism, and occasionally against the entire Enlightenment ideal of a better future.
Many critics (including George Orwell) have commented on the inability of authors to imagine Utopias where anyone would actually want to live. If no one can imagine a Future where anyone would want to live, that may drain off motivation to work on the project. But there are some quite understandable biases that get in the way of such visualization.
Going into the details of Fun Theory helps you see that eudaimonia is complicated - that there are many properties which contribute to a life worth living. Which helps you appreciate just how worthless a galaxy would end up looking (with very high probability) if the galaxy was optimized by something with a utility function rolled up at random. The narrowness of this target is the motivation to create AIs with precisely chosen goal systems (Friendly AI).
Fun Theory is built on top of the naturalistic metaethics summarized in Joy in the Merely Good; as such, its arguments ground in "On reflection, don't you think this is what you would actually want (for yourself and others)?"