How has peer pressure affected your life?

I'm 15, and have come to realize that I've never actually cared what other people thought of me. I feel disconnected now, as I am unable to understand why other people care so much. Why they do things just to impress others. Why do they never stop and ask themselves, "Is what I'm doing making ME happy?" That was phrased rather bluntly, and perhaps sounds selfish, but I already realize that life outside of school really has nothing to do with how you slaved away to impress your friends, and has little impact your life actually plays out. Hard work in school is what pays off. "Those who care, don't matter, and those who matter, don't care," as Dr. Seuss phrased it. I ask for the general opinion, however. Because the general wisdom is of more perspective than my own. So, I must ask... Why do others care so much? How has this impacted YOU?