I think all of the best nonfiction that has ever been made comes from the result of someone who can't stop thinking about a certain topic - a very specific aspect of a certain topic in some cases. And second, they got really good at figuring out what they had to say about it.
Merlin MannPriorities Are Like Arms; If You Think You Have More Than A Couple, You’re Either Lying Or Crazy.
Merlin Mann