Wow, this is kind of a big question. I don't know if I'll be able to answer it in the fifteen minutes I have available to write this blog entry. All the same, I'll give it a whirl.
I suspect my reason isn't so different from anyone else's. This is the way my mind works. It spins stories up until the moment I fall asleep at night. As I go through my day, little things jump out at me and scream, "Me! Use me in your story! I could be good!" When I talk with people I wonder about their histories and what pivotal moments might have brought them to their current point. Sometimes I devise histories for them. When I was a kid, if I wasn't writing plays or directing my friends in elaborate games of make-believe, then I wasn't happy. I gobble up stories in the form of books, dances, music, and art and my brain churns them into each other and spits them out into something new. Much of the time I don't even realize I'm doing it, until a morsel of an idea emerges and we're off to the races again.
The truth is: I'm full to bursting with stories and if I don't get some of them out, then I start to go crazy.
And crazy me isn't a pretty thing.