I read a lot of books and articles on things like brain science, psychology, productivity, and memory. One thing that tends to come up often as I read is that the brain sees any incomplete task or goal as part of your to-do list. Even if you arent consciously thinking of it, anything that you have left undone lingers in the subconscious and will periodically torment you until its finished.
This applies to everything in your environment including unpaid bills, an unfinished presentation for work, unread books, a Tivo full of unwatched shows, or hobbies that you started but never did anything with. You may not have these things on your immediate to-do list (in fact, they may not have been important to you for months or years), but your brain does.