Why are humans so mean?

i dont get it. humans are mean. its like hardwired into our brains. like to pre judge peopel, it happens automaticlly. Animals dont behave this way, why do humans? was there ever any advantage to doing this during caveman times or soemthing? was there any use of it before? it's so frustrating. especally for peopel who have to deal wiht it everytime they go outside.