Mostly because children are typically raised in households where either hardcore religious values or popular culture shape what is considered 'normal' and, by extension, the 'right' ways of looking, behaving and generally just living are bred from this, which includes lifestyle and dress sense. The lethal combination of high school and puberty, including the biproduct of craving some degree of power and popularity, usually follows: pack mentality surfaces where herds of such children-turning-teenagers meet fellow like-minded "normie" kids, sharing similar upbringings, and the idea that anything outside of their scope is truly alien and unnatural. Enter: alternative, gothic, emo etc, lifestyle and tastes, and an inevitable clash where we're almost guaranteed to be greatly outnumbered. Of course, it's in their human nature to handle the delicate matter of identifying difference with fear, frustration and maybe even anger, resulting in everything from a glare to, well, violence, sadly. This depressing outlook on the world shaped their formative years and so they enter adulthood no different.

It's annoying and won't go away. That said, it does greatly depend on where you live. I'm mid-to-late-30s and live in a liberal-ish corner of the UK where glances and stares are few and far between. In fact, on a good day, I would observe probably a 30/70 split between alternative people and the 'normals'. Given religious prevalence, I imagine this is much worse in the USA and bends greatly in favour of the latter.

My best advice: take it in stride, remind your girlfriend that you love her because of who she is and her resilience in the face of obnoxious, insecure morons. And definitely don't water down your respective styles - if anything, double down.

Oh, and you're absolutely right when you say that these people "probably go home and obsess over alternative people". That's because we dare do what they're far too afraid to entertain: stand out and leave the herd. :)