I think the basic reason why people react so to music, is that MOST music has emotion tied to it. This is regardless of weather it's opera, reniscance, classical, country, blues, rock, metal, rap, or some pop. This is exactly why I believe you get such an agervated reaction from people when you tell them, 'hey man, your music SUCKS!'. They have emotion and love for that music, some more than others. Just look at videos on youtube and look at the comments. Some people get down right hateful on there and for no reason. I don't like country, never have. But I'm not going to go around smashing it just because I personally don't like it. If some one else wants to listen to it, that's fine, that's their choice and it doesn't effect me in any way. I've noticed where I work changes in attitude and work ethic to go along with music. We are allowed to listen to mp3 players while we work and we get paid based on how much we porduce. When I get some Metallica, Hatebreed, or Amon Amarth come on my player, for some reason I will almost always speed up and be more agressive in the production process. It never fails. In fact, one day I listened to Metallica for the whole 10hr day, that's right, Metallica for 10hrs and I never heard one song repeated. That day we broke our team record and I attribute at least some of it to the music I was listening to.
Plain and simple, music is a powerful thing.