Nu mi-e clar de ce nu se numeste Happiness Theory tho
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.
Fun Theory is also the fully general reply to religious theodicy (attempts to justify why God permits evil). Our present world has flaws even from the standpoint of such eudaimonic considerations as freedom, personal responsibility, and self-reliance. Fun Theory tries to describe the dimensions along which a benevolently designed world can and should be optimized, and our present world is clearly not the result of such optimization - there is room for improvement. Fun Theory also highlights the flaws of any particular religion's perfect afterlife - you wouldn't want to go to their Heaven.
Finally, 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.