1) Music seems to have the closest link to my emotional state of anything short of romantic relationships. It's rather trivial to hack my emotional state into mellowness with a song like http://www.youtube.com/watch?v=paeNnR33i5Q, or into an aggressive upbeat state with something like http://www.youtube.com/watch?v=ZO6giM9UAv0, or appreciation of civilization with http://www.youtube.com/watch?v=FAvQSkK8Z8U. No other form of art can do anything near as much for me - most of them I appreciate on an intellectual level, but music is heavily emotional as well.

2) I've always listened to a lot of it, which means that there's a whole mess of assorted nostalgia attached to it.

3) Similarly, silence sounds unusual to me. I don't mind a bit of it, but too much(at least, in a context where music is possible) creates a distinct sense that something is missing unless I'm engrossed in something else.

4) I find that a minor distraction actually improves my focus sometimes - giving my brain a B-plot, so to speak, short-circuits a lot of wandering thoughts. Music, especially music I'm familiar with, gives my brain something to latch onto when it's idling that can easily be pushed back into the background.