Why do people say nice things to criticize you and hurt you?

one day i was at school and this REALLY skinny black girl with short hair basically yelled & shouted at me "YO HAIR IS CUTE GIRL !" and i KNOW that she didnt mean it . she was trying to crush my confidence and make me feel bad but trick me into falling for it so she could laugh at me , but she laughed at me regardless . . i mean there was nothing ugly about my hair ? i mean my hair was down and straight ? i hate it when girls act like that . . why do they do tht ? ALOT of black girls at my school do that . . and i think that its very disrespectful . & also why do some girls tell you your really pretty when they dont really mean it but they are just trying to not hurt your feelings but they were basically lying and think that your ugly? why do they do that? i mean when people giv compliments to other people its hard to tell if they mean it or if they are just lying . . its like they are trying to make fun of you without ACTUALLY saying the things to make fun of you . . i just dont get girls these days . . does anyone get what im trying to say ? you guys would really be a big help answering my question ; and when people giv me compliments should i just nod instead of saying thank you ? because its hard for me to think they really mean what they say basically .