I haven't always wanted to write. My dream jobs as a child were, in no particular order: astronaut, oceanographer, soldier, and inventor. I realized I wanted to write at 13, when my school library published a classmate's book. Out of nowhere, I felt If he can do it, I can, and I've geared the rest of my life toward writing.
I write because writing is more active than reading. Reading, to me, is like watching sports. I enjoy watching, sure, but I prefer to get in the game.
I write because I feel I can express myself best in writing. Planning what to write and revising help test my logic before I spring it on people. If I didn't write anything, kept everything to myself, I'd go stir crazy.