I have a few points on why I think people find different forms of music pleasurable:
Good music makes me want to move my body, sometimes it feels involuntary, and the movement that results seems to release pleasure sensations. Dancing is widely accepted as being a vehicle for endorphin rushes. There is also a link between pressure on the balls of feet and pleasure release. Obviously this doesn't apply to a lot of music and to people who don't dance or move to music.
2. Giving more power to words:
Music can emphasize written words. So when a musician performs a song, they not only have all the powers of vocal poetry but also the added emphasis of shifts in pitch, tonality and rhythm. (Personally, this one is particularly important to me as I primarily enjoy music that has either poetic language or deals with social histories through song).
3. Marking time:
Because most music is clearly structured in terms of rhythm, this is effectively another method of marking or observing the passage of time. In my opinion, it appeals to the human brain because it is another way of trying to understand time, make sense of it - another form of time-structure.
We're emotional creatures. A lot of music involves the aural expression of emotion. In some passages of song you can hear the strained sounds of pain or sorrow, the elated sounds of happiness or mania, in the singers voice which can in turn stir similar emotions or sympathy in the listener. This doesn't apply strictly to vocal music, as many instruments seems to mimic or emulate vocal sounds. I think a great example of this is how people will often describe a 'haunting violin piece', or possibly akin to the sound of a cat being strangled. Through music, humans try and capture the sounds and tone of human emotion. Shared emotions reinforce humanity and the common ground among people.
I guess I'm essentially coming to the same conclusion many have made - it's yet another form of communicating emotions & ideas, another manisfestation of language. Communication seems to be one of the major sources of human pleasure.
But the thing that still fascinates me is how different scales, chords and intervals can affect different people to varying degrees of pleasure/pain. Most people would find loud machinery unpleasant to listen to, yet there are musical styles (whether we count them as music or not is another whole debate) that aren't too far removed from the sound of heavy machinery or factory noise.