According to Merriam-Webster:
fun: providing entertainment, amusement, or enjoyment
For me, writing is part entertainment, part empowerment. I do it because I like it (or else nothing would keep my butt in that chair hour upon excruciating hour), and I do it because, after the pain of giving birth to words, I feel like I've done something worthwhile. Something that makes me worthwhile. If all that effort results in something that's bigger than myself, something other people can take and make their own (and this something came from my head and heart and entire being), I feel something akin to pride but closer to pure and unadulterated happiness.
It's like seeing your kid go off an find a cure for cancer. Or else, go off and fall in love and make someone very very happy. It's the gift that keeps on giving, it pays you back a hundredfold. The process of making it hurts, sure, and sometimes makes you want to throw in the towel, but in the end, the pain only makes the joy of having created something good more worth it.
Of course it's hard. Creation is always hard. Even God rested after creating the universe.
All we're doing is stringing a bunch of words together in new patterns of meaning. But that doesn't mean it can't also be sublime.
Fun? Sometimes. The process of writing is too complicated to be pinned down to just one word.