I think every human being has a need for self-expression. Some of us do this by practicing some kind of art. I have tried writing, drawing, painting--and now composing, and I find that they are very much alike. To do them they all need some sense of art, and some knowledge about craft, but via them we all do the same: try to express our feelings, our inner workings, and make ourselves feel that we created something; something that's new and different from the things that were already there.
When I read a book, and see how well it's written, I instantly feel an urge to write something--I just want to make my own. The same is happening with music--I just want to try and create something that is mine and that I can listen to, and pretend that it has any kind of artistic value :P Anyway, as far as I see, a symphony tells the same things as a poem, or a painting or a novel--it's a bit of the author's soul crafted into something considered "a piece of art."