When most writers are asked why they write, the standard answer is "I have to" or some variant. One thing you will quickly learn about me is these types of answers never satisfy me. I always have to know why. In this case, why do I have to. There are as many answers to this as there are writers, but here's what I figured out.
After I got out of the "I want to be a faerie princess" stage, I wanted to be a paleoanthropologist. I loved the idea of working through the puzzle of human evolution. That is until I decided the actual field work was the last thing I wanted to do. Next I wanted to be a geneticist. I loved figuring out the puzzle of the human genome. Again, that is until I decided I hated lab work. Next came psychology. I loved puzzling through the human psyche. That is until I decided I would want to slap most of my clients.
The common thread is I love puzzles, but I hate what I have to do to collect the pieces. Not so with writing. Writing is one big puzzle, but I don't collect pieces. I create them. If X isn't working, I can toss it out or create something to make it work. I love the actual process of writing and love working with the pieces I create.
Another common thread is I love puzzles that deal with our very humanity. That is what books are. They tap into our collective unconscious and touch us on a profound level. Writing is not just dealing with any puzzle. Storytelling is encoded in us and one of the things that makes us human.
I am a paleoanthropologist who studies what environmental pressures affect my story. I am a geneticist who explores the various building blocks of my story and sees the results of various combinations. I am a psychologist who need to understand my characters and why they act the way they do.