Why we write/what we write

I'm exploring why we write and what we write when we do.  When not given a prompt or a picture to base our writing on, why do we choose to write what we do?  For me, I mostly write about my background. Experiences I've had in my past that shape me in good or not so good ways.  We write what we know right? Here are some essays online from famous and not so famous writers about why they write: http://whywewriteseries.wordpress.com/