Seperation Of Music

Since when did music separate everyone socially?
I realize a lot in other sites (i.e. facebook, myspace, myyearbook etc.) I see people writing things like:

"If you listen to "this" you're "this" or if you listen to "that" you're "that"

Does it really freaking matter???

music is music.
It doesn't make you anything except someone who listens to music and if you think otherwise you have a problem...seriously.

For example, I listen to Avenged Sevenfold, Plies, Tokio Hotel, Justin Timberlake
and at the same time I can listen to Bebel Giberto and Utada Hikaru without about
worrying how every picks what I am in their eyes.

It all really boils down to this clique crisis that most teenagers are going through (yes
I am a teenager and I can address the group as "teenagers" as if I don't belong)
I experienced it in high school, just like every one else; the unusual reality of being categorized as if you were meat in a grocery store.
Since I dressed a certain way, listen to certain music, and I wasn't typical I was thought of as "emo" or an "oreo"
When honestly, I simply wasn't a stereotypical black girl at my school that wore short skirts and listen to rap and R&B and/or adore Lil Wayne.
Another question?
Do people realize that when you leave high school no one cares if you were prom queen or quarterback or the emo kid or the preepy chick or the Picasso wannabe, if you still embrace the whole high school cast system when you graduate you are going to walk into problems.
After the moment you leave the stability of high school you are none of that anymore.
So this comes to my last question, which is what I want thoughts on.

Why does it bother us so much in those dreadful four years?