As I imagine is true of many, I began writing as a teenager as a coping mechanism. I wrote poetry to deal with depression, and growing up in an emotionally abusive household. Living in an environment that was not kind to any show of emotion, I learned to express my emotions through writing. I couldn't be punished for expressing how I felt in a notebook that my parents wouldn't read anyway (if anything, they probably would have made fun of me if they knew I was writing poetry). This was the reason I wrote for 10 years of my life.
Then my motive for writing changed. My best friend had been telling me all about how he had been spending large amounts of his free time writing. Prompts, challenges, poetry, collaborations, short stories. Have spent 10 years writing poetry, this sounded like a way to possibly share my work with others. And once I started writing beyond poetry, my reason for writing was the enjoyment it brought me, and the self improvement I felt.
Suddenly, I did not write for almost 5 years. That stretch was filled with partying, and career. When I wasn't working, I partied. When that phase of my life ended, writing was never something I had thought about. My life changed so much in the 5 years I was away... but now I am back. And not partying every free second of the day.
So why do I write now? I have an amazing family and a wonderful, promising career. I no longer need to write to show how I am feeling (although expressing/showing emotion can still be sometimes difficult). Those reason are behind me. It is still about fun and self improvement, but I also believe if anything I write helps anyone in anyway, then it is the greatest way I can think to help.
Hopefully someone reads one of my works and can escape, if only for 500 words, from whatever troubles them in their life. If I can make someone laugh, cry, or feel anything from what I've wrote, I feel I've left a positive impact on the world. I believe that if more of us tried to leave positive impacts on those around us, we would all be much better off.
And that is why I write.