Ironically, I've just finished working on a fiction project today myself! It was on the back-burner for many years then one day I thought 'the book's not gonna write itself, I better do it!' and now I can't believe it's done.
Some things are actually passion but get buried underneath as we get on with life. Once you start paying more attention to those thoughts, they can take on a life of their own. A bit like fanning a bunch of tiny flames can produce a bonfire!