Why are people so fake?

People try to pretend to be nice but the second your alone you hear them gossiping about you, calling you a slut when they don't even know you, saying they are scared of you. Its so ironic that this humongous ape at school said she was scared of me when I haven't even said one word to her. I accidentally bumped into this preppy guy when walking to my seat, I said sorry and then sat down. I could hear them saying "ew." and talking about it. I can hear people talking about me and they think I'm def. Its annoying, why do people even bother trying to be nice anymore when they just turn around and do the same bullsh*t they always do..?