Just a question.
Tricky topic. The most concise answer, assuming you want something more specific than "gives us pleasure," is probably "we don't really know yet."
Here's some reading:
http://www.amazon.com/This-Your-Brain-Music-Obsession/dp/0452288525
http://www.amazon.com/Musicophilia-Tales-Music-Oliver-Sacks/dp/1400040817
The latter is less of an attempt to answer your questions and more a gallery of strange neurological conditions related to music (if you've read Sacks before, you know what to expect) but is pretty interesting. And I haven't actually read the former, so I thought I should throw in one book that I have read.
The brain has built-in reward systems for pattern matching because it is a valuable survival skill. If we never evolved a manner in which to recognize the sound of an animal or a rhythm such as an Earthquake, the wind, the ocean, etc. etc. we probably wouldn't have survived.
Music is especially pleasing because instruments are tuned to be in harmony with each other, making the pattern very obvious.
music is loved.
its like asking why we masturbate to pixels on a screen.
symbolic of something which was once useful to survival.