The fact that there is more old good music than I can possibly learn and perform out there already doesn't mean I don't want to hear your new music. Learning and performing music is partly about the joy of discovering something new. You are new. And your music is a reaction, conscious or unconscious, to everything that has come before it. That is interesting. Your place in the long march of history is the same as mine, so what you have to say is interesting to me.
Personally, I like composing for the same reason I like performing music. It's a way to be personally involved with an act of creation. Everybody likes that, whether it's baking a cake or writing a sonnet.
I'm not likely to ever get paid, but that's okay with me. I can write some music, or I can watch TV. I've got the time. I might as well use it to do something interesting. Death before TV. What a boring life.
As for self-promotion, go hang out on the conductors forums. Music directors are always looking for recommendations to fill specific slots on their concert programs. (I need a piece about 5 minutes long for string quartet plus a trumpet to fit into program of modern work…) If you offer them your piece, which happens to fit their need, you're self-promoting, but you're also solving a problem they have. Which feels pretty nice, actually. (: