It's a strange question to ask yourself.
You never really think about why. You just kind of do it. You write because you have to, for school, for work, or for anything else. You may write because you just feel like it, as in a journal.
Why do I write?
I write because it helps me get my frustration out. Too many times it's been the case that I have frustration, anxiety, and general feelings of being upset built up inside of me, and I feel if I don't get them written down in a ranting sort of letter, I'll explode. I like to write these feelings, because obviously people get tired of hearing it all the time. My notebook and pencil never get tired of me.
I write because I can. I can write about whatever I want to. I can visit anyplace I want, do anything I please when I write. It's just a nice escape from reality.
I write because I want to improve. That's the only way it can be done, right?