I think about this question a lot because the answer changes depending on what's happening in my life at any given moment.
The compulsion to write is often spurred by the desire to work through a situation. The need to chew on the details and digest their meaning.
I may not share the details here for all the web to see, but the process of working through the thoughts generated by events is important for me.
When I was ten years old, my mom bought me my first journal. She got my six-year-old brother one as well. That journal was a cheap spiral bound notebook. It had a slate blue plastic cover that I decorated using a silver paint marker.
I still have that notebook - it's buried in a box somewhere in my house. That's where my ten, eleven, twelve-year-old self lives on between plastic covers painted with silver hearts and my name in an effort to keep prying eyes deterred from breaching my privacy.
I don't remember well what I wrote, but I can imagine I described my crushes (I was pretty experienced at crushing since I started at six), some parental angst, likely occasional friendship difficulties, with the odd mention of school, where I spent most of my time.
I'm not sure whether mom really intended to get me interested in journaling or if the purchase of those notebooks were just a whim, but I have had an "active" journal ever since. I've even filled some of them up.
It seems wasteful not to fill them all up, doesn't it?
But I haven't. And I doubt I ever will.
Too many times I've closed a journal halfway through never to open it again because that part of my life ended. I needed to move on, and buying a new journal was a symbolic way to do so.
I've gone many months without journaling at all, often because I couldn't find the words. Sometimes I just need to let events of life that have boiled over simmer down and cool off before I can dump my thoughts out onto the page.
Writing fills a need in me.
I don't think I knew that need existed back when I was ten and staring at the blank pages of the cheap notebook my mom bought me. But I definitely felt the excitement of the blank pages and the possibility that was waiting for my discovery.
In his book, On Writing (affiliate), Stephen King shares his belief that every story exists before it is found. That stories are like fossils that writers - literary archaeologists - carefully and methodically search for and uncover. Sometimes with great care and concern for preserving the quality of the story. Sometimes with less regard. (I'm adding in a little bit of my own interpretation.)
This idea resonates with me because I've had the same compulsion to write the stories of the people that live in my imagination as I do when I am working through the process of writing my own story.
So, why do I write?
Because I need to.
I don't know how else to answer. Even when I can't find the words, the need doesn't go away. And eventually, the words always come back.