1.) Identity. To be somebody. A "writer." A badge. A certificate of existence. Now: what to write?
2.) Hobby. Using time. Filling up space in one's life. Doing something. Macrame.
3.) Money. Write a best-seller and retire from life.
4.) Therapy. Personal dilemmas laid out on a page. Journals for an unseen shrink.
5.) Message. An announcement. Having something to say and compelled to say it. A discovery.
6.) Change. Writing to change society, or the culture, or the neighborhood. Or the world.
7.) Immortality. Creating the masterpiece that will live forever, honored by all subsequent generations. A place in future generations.
8.) God. To understand or commune with the universe.
Obviously, there's overlap. Many individuals write for more than one reason.
What's your reason?