Then there's the fable of the ant and the grasshopper. Work makes you dull, but fun makes you dead. When you think about it that's not much of a choice. There is reason to be circumspect about fun. It can be frivolous.
Time is precious and using it nonproductively can be wasteful.
While children are supposed to play, we think less of adults who spend too much time playing. When it comes to religion, which is supposedly the really important stuff, it's not supposed to be fun. I personally have problems with that since I believe fun and play are vital to healthy human living. Perhaps the dullness that stalks funless Jack is not merely boredom but dullness of mind and dullness of soul. Perhaps in that sense real religion should be fun.