I used to think I did it because it's the only thing that I'm good at. And although that's somewhat true, I don't think it's the actual reason.
Another reason is that I get depressed when I go through a period of time without writing.
But here is the most significant reason:
I have done a writing exercise called "I remember" a number of times in classes where I was a student as well as a teacher. For the exercise, I would write "I remember" before each sentence and write about memories from my past. By doing this exercises, I have thought of things I have not remembered for years. Some of them I haven't remembered since they happened.
And this is related to the reason why I write. I never know what's going to be transferred from my brain and onto the screen (or the page). And it is always exciting to find out.