In writing fiction, you are doing something not everybody can do. Practicing an art form you love.
Ideas leading to other ideas. My idea from The Trouble With Mother led to The Cherry Tree.
Coming across something badly written and knowing not only can you do better, you are doing better. It is a massive positive boost for the ego. You can often learn more from a bad piece of work than a good one, if only not what to do. The trick is to ensure the bad piece is not one of yours!
Enjoying making up your own world but getting the rejections that prevent you making some money from it! Incidentally ignore Johnson’s quote that “nobody but a blockhead writes for anything other than money”. You’ve got to write for the love of it. It’s that which gets you through the rejections.
Having writing fiction dismissed by those who can’t do it!
Not having enough time to develop all the ideas you’re getting or discovering the ideas lead to a literary dead end and weren’t as good as you first thought.
Being dismayed by the fact the badly written piece did get published, whilst your better work is still languishing at home!