I can't help it. Let me overhear an intriguing scrap of conversation, or read a bizarre headline, or learn about some new-to-me quirky historical fact, and my brain will start constructing a story around it.
It doesn't matter if I ever write the story; I just seem to have an automatic reflex to outline the possible story in my head, maybe come up with an opening line.
It's like a marathon runner doing a little jog-in-place. It keeps me in shape, preps me for the big race. Only in the case of writing, I never know which little jog is going to turn into the marathon until I'm well into the race!