My motivation is fun. I read a lot and I have the secret ambition to be an actual writer one day. But I'm not really flexing my writing muscle here. English isn't even my second language.
I just think of funny story ideas and put them into text. It might change, but right now I'm basically writing sketches that end with a punchline as a lazy out.
If you need a simple formula for writing stories, check this out:
Holy crap! That was awesome. Thank you for sharing the info graphic.