Where Does It Come From?

Writing and love come from the same place -- the heart. You can't write from any other place, which makes love the perfect analogy for writing. You write every day because you love the effort, the pain and the time you have to give to make it work, to make it worthwhile, to be successful. And let's be honest, success in writing has nothing to do with how often you are published or how much money you make writing. Plenty of hacks have made millions of dollars, no matter in what form they are writing -- novels, newspaper columns, scientific articles, especially screenplays.
If you want to write you have to do it from the heart. It's the only place you can love from or write from.