Because I have to.
I've always told stories, even as a very small child. It used to take me two hours to walk come from school - which was 5 minutes away by normal-person walking speed - because I kept picking stuff up off the ground or getting lost exploring hedges and fields, constantly making up stories about what I saw. My parents never had any trouble making me play outside, because I'd just grab a stick and run around pretending I was a knight on an adventure.
Even now, as a grown-up - I don't sing in the shower. I talk. I tell stories. If I'm left alone for more than a moment, the stories I'm constantly telling myself in my head, I start telling them out loud.
I used to write prose a lot, until one day (quite late, I admit) I stumbled on the idea that I could make comics, and then I just sort of never stopped. The art and the writing aren't really separate for me when it comes to comics, because they both serve the same purpose - to tell the story.