Why does society judge quiet or shy people so harshly?

It is because people tend to fear the worst these days when it comes to quiet and reserved individuals. Often, those people tend to keep things quiet and bottled up until they can't contain it any longer, and since they keep to themselves, there's no way of knowing what's going on. The resulting eruption can range in severity from out-of-control screaming to physical violence. Events like the Columbine Shooting are often attributed to loners and social misfits (though in reality, the primary boy who masterminded the Columbine Macre had more in common with a serial killer than a shy, picked-on, WoW addict).