Which brings me to my first point, which I will allow someone else to make for me. You write or you don't. The curse of this profession is that it's so much more fun being a writer than it is writing. I've never found another quote on the subject that was more true. Being a writer is fun.
Writing is hard work.
They usually never do finish the book. After all, starting a book is easy, finishing it is hard. And getting it published is damn near impossible. When I first began writing short stories, I was still working as a daily newspaper reporter. I figured I'd try writing fiction for 10 or 20 years and if nothing happened in that time, then I'd spend my spare hours happily doing something, anything, else.