It's a bit like asking someone with a gambling addiction why they play poker.
When an idea makes a nuisance of itself, more often than not, it's in movie form. As I explore whether it's worth a toss, characters introduce themselves, or a good movie title, or sometimes a scene that I think would be amazing to see with a bucket o' popcorn and JuJu B's on my shoe. As the idea grows and refuses to leave me alone, there's a point where I'm kinda like Fred Flintstone when he'd go all meth addict and say, "Bet-bet-bet-bet-BET!"
Then, there's no stopping me. Time to pawn my DVD player and lay down my bet by writing that damn thing. At the end, sure, I'm proud that I finished it, but there's probably some sense of relief that it will now leave me alone to some degree.