I think there was already a thread for this..but I'm too lazy to look.
I write music simply because I love doing it. Music is usually the only place I can turn to when I'm down or need a place to vent. Music is pretty much the only thing that makes me happy in life, besides my girlfriend, friends and family. It's a place for me to tell a story, which I love doing, in a form that I love most, music. I also enjoy my random outbursts of "This is going to sound sick!" and other phrases that are similar lol
There are more reasons, but I'm too tired and lazy to think of them.