It seems a bit of a cheat to use a movie clip to answer a question, but then again this site is intended to be one of media criticism and analysis, so sort of tracts. And to be perfectly frank, my answer is not dissimilar from the one provided by the video above.
I write because I am compelled to. I get stories, poems, ideas, or just plain feelings in my head from time to time, and I can do nothing else but put them to paper or screen to get them out. I write because I always have and always will. I will probably die with a story in my head waiting to get out. I write because I cannot imagine what I, or my life, would be without the release and act of writing.
As plain as can be stated, I write because I can and I want to. What other reason could I possibly need.