Hormones, girls that are prettier than us and boys that reject you.
Well, for me it was always because I had a very destructive friend who would tell me that my face was ugly, my hair looked horrible, my outfit sucked, my boobs were tiny and my shoes were gross. Everyday, I would hear these messages and it wasn't long before I started to believe them. When I finally closed her out of my life, I felt so much better about my appearence.
Girls that r prettier then u
Others putting you down.
Blame the media and other pretty girls around them:)