I used to be a prolific writer, more so than now. In college I published a zine, wrote for the newspaper, and compulsively kept a journal. The other students on my dorm floor would complain about hearing my typewriter at all hours of the night (this would be the '80s, when computers were expensive and I had to go to the library to use them). These days I mostly write legal opinions and emails, but I do have a blog, and I continue to do reviews and previews for the local alt-weekly.
I don't know why I keep writing, so I guess I "have" to as well. I'm far more comfortable expressing myself in print than verbally, so that's a big part of it. I've often wished for a delete button in conversation.