Music is the purest expression of emotion possible, but expression of emotion is not what really always drives me (solely) to compose.
When I first started composing, it was soon after I first started getting into Classical (and film) music. Before then, I listened to rap. (I mean seriously). I'm not too sure if I liked rap, but needless to say, it didn't implant any musical desires in me. However, upon listening to Classical, loving it, I just felt such a desire, a NEED, to further express my love of this music. I realized how amazingly music can speak to our spirits and hearts, and I just wanted to be a part of that, to utilize the most abstract yet accessible tool humans know of.
Once I started composing, it wasn't a matter of, "Oh, this sounds terrible. I WANT TO BECOME BETTER. I WANT TO SOUND LIKE *such-and-such composer*." No, it was more along the lines of, This music lacks emotion. How can I conjure that special feeling I have when I listen to music I love?
Nowadays, I'm very aware of why I love writing music. There are two facets. The actual composing, during which I am almost animalistic, thinking only of the music, unaware of my body, my heart and mind solely on the task before me, making this music amazing. Yet at the same time I don't do things very much on purpose. It's almost like those free-write sessions, where you let your pencil write whatever is in your head, and don't stop. It's a flow of thoughts and emotion in the form of music. So, on one level, I love music for the actual composing, the feeling that I'm totally losing myself to this composition, this creation.
Then there is the completing. The finishing and listening to the final product. Believe me, at that very moment, that one time where you input the last note, and you know you've created something awesome--nothing can beat that feeling. It's the best feeling of the world. Just a feeling of utmost pride and satisfaction and joy. I compose for this feeling. If I don't feel this feeling--I just feel "Eh, it turned out not too bad," then it almost wasn't worth it.
A third thing that makes me love what I do is reaction. When people listen to my music, and really love it--not because they want to make me happy--but because they heard it and really want to express how great they think it is...that's an awesome feeling as well. When I get highly praising reviews I just feel happy, extremely happy. It almost feels like I've changed someone's life through my music.
I will never stop composing :]