Haha, I think I'm somehow super passionate about the game, it generates ideas, then I work on them, which makes me even more hyped, which generates more ideas, which makes me want to work on it more, and it becomes this like positive feedback loop or something. And then I get all excited and I just HAVE to make these ideas a reality (which hopefully end up being awesome).
Also a new project is always fun, clean slate to do anything you can dream of and I just love rapid prototyping (one of my strong skillsets I've been told, I think).