The satisfaction of knowing you are writing your own work. I’ve nothing against ghostwriting but you miss out on the fun of writing getting someone else in to do it for you!
Coming up with a turn of phrase that fits perfectly. Likewise scenes that you’ve seen in your head working out wonderfully when you’ve got them down on paper.
Getting ideas for more stories!
Feeling what you’re writing is awful. Despite knowing all writers feel this way and the thing to do is put the work aside and view it later with fresh eyes as it may not be as bad as you think, the feeling in the gut is still dreadful.
Getting annoyed when you know those claiming to have written a book (but actually used a ghost writer) do well with that book!
Struggling to get phrases and scenes out. Keep going. I don’t believe in writer’s block. When the struggles come, get anything on paper. There may be something in it you can use later. I learned ages ago whenever someone makes something look easy, you can guarantee that same someone has worked damn hard over years, decades even, to get the experience to make things seem easy. There are no shortcuts.