There is a great book 'This Is Your Brain On Music'. I would recommend buying it. The eighth chapter talks about some theories on why we like the music we like. The first theory is that we are more attracted to things like what we heard when we were in the womb. Inside the womb, surrounded by amniotic fluid, the fetus hears sounds. It hears the heartbeat of its mother, at times speeding up, at other times slowing down. And the fetus hears music, as was discovered by Alexandra Lamont of Keele University in the UK. She found that, a year after they are born, children recognize and prefer music they were exposed to in the womb. One other has to do with pitch and psychology. Pitch can also play into preference for that matter. Some people can't stand the thumping low beats of hip-hop, other can't stand what they describe as the high-pitched whininess of violins etc. Part of this may be a matter of physiology, literally, differet ears may transmit differet parts of the frequency spectrum, causing some sounds to appear pleasent and others aversive. There may also exist a set of psycological associations, both positive and negative, to various instruments. There is a theory that has to do with the complexity and simplicity of music. A piece that is too predictable is often undesirable, but one that's too adventurous and unfamiliar is also undesirable. Thus a balance has to be struck between the two and different people have different balance points.The orderly relationship between complexity and liking is referred to as the inverted-U function because of the way a graph would be drawn that relates to these two factors. Meaning try to imagine a graph in which the x-axis is how complex a piece of music is (to you) and the y-axis is how much you like it. At the bottom left of this graph, close to the origin, there would be a point for music that is very simple and your reaction being that you don't like it. As the music increases in complexity, your liking increases as well. The two variables follow each other for quite a while on the graph-increased complexity yields increased liking, until you cross some personal threshold and go from disliking the piece intensely, to actually liking it quite a bit. But at some points as you increase the complexity, the music becomes too complex, and your liking for it begins to decrease. Now more complexity in the music leads to less and less liking, until you cross another threshold and you no longer like the music at all. Too complex and you absolutely hate the music. The shape of such a graph would make an inverted U or an inverted V. The last theory, and most fascinating (to me at least) has to do with familiarity.The types of sounds, rhythms, and musical textures we find pleasing are generally extensions of previous positive experiences we've had with music in our lives. This is because hearing a song that you like is a lot like having any other pleasant sensory experience-eating chocolate cake, fresh-picked raspberries, smelling coffee in the morning, seeing a work of art or the peaceful face of someone you love who is sleeping. We take pleasure in the sensory experience, and find comfort in its familiarity and the safety that familiarity brings. Safety plays a certain role for a lot of us in choosing music. To a certain extent, we surrender to music when we listen to it. We allow ourselves to trust the composers and musicians with a part of our hearts and our spirits. We let the music take us somewhere outside of ourselves. To sum it up, people like music that they are familiar with in various ways, be that familiar emotions, concepts, complexities or simplicities, timbre, rhythm, or other factors. These theories could have many interesting repercussions. Fans of metal, classical, and avant garde obviously have a high tolerance for complexity, while fans of rock, blues, pop, and hip-hop don't like to step outside their musical complexity/simplicity comfort zone. The familiarity theory can imply that people like music that expresses emotions and ideas that are familiar to them, be they positive or negative (they'll still find comfort in it and probably enjoy the music). Metal for example, under this theory, with its emphasis on anger, chaos, alienation, and (in cases) philosophy, appeals to people who are familiar with these things. Which is to say the majority of metal fans have often felt, or feel angry and alienated among dozens of other factors (traumatic childhoods, experiences with violence, etc). This also could explain why fans of classical and metal music tend to be intelligent and/or like minded. This can go likewise for the dumb and ignorant masses. Dumb and ignorant music, for dumb and ignorant people (though we already knew that). I heard a theory from someone that people like music based on the way it vibrates the blood in their body. This reminds me of the water crystal experiments by Masaru Emoto, considering blood contains water as well as the percentage of water in the human body, so perhaps certain styles of music appeals to certain people because it creates crystals that are vibrated apart are specific frequencies that appeal to those individuals.
On a side note it could have said x = complexity y = popularity, but it is revised to allow for personal interpretations of the music not mass acceptance. Implicitly, the book is reinforcing the arguement that some music is too complex for some individuals. A teenager who only listens to rock or pop acts will find Wagner to be on the downward arc of the graph curve (too complex). Someone who regularly listens to Classical or Technical Death will plot the music somewhere at the peak instead (again, because familiarity in the form allows a greater understanding). The theory allows for cognitive interpretations of music, it even implies that some music can only be enjoyed through this level of engagement.
Anyway just thought it was interesting and you might get a kick out of it.
Come manifest yourself in front of me, that I may pee on you......
I should really send you cookies or something for being so thoughtful and helping people to realize "what they need to do" 
oh jeebus.... *nervously walking away*
