This is the big reason that takes most of the blame. Why? Because most people are insecure in some way or other, and this is even truer of teenagers. It’s easy to say the main motivation of all bullying is insecurity, but sometimes it’s not. In some instances, it can be an influence, but not the big reason.
Insecurity is the reason when people put others down to detract from their own failings. It can be based on insecurities about the usual sorts of things: looks, social standing, friendships and relationships, wealth, talent, skills, etc….
If someone insecure punches you, patting him on the back and telling him he has many good qualities and shouldn’t feel insecure will not solve the problem. Unless you feel it’s a problem that your nose isn’t broken, or that you’re just not bleeding enough and have too few bruises.
If someone insecure bullies you through words or relationships, it’s also bad to directly point it out. Suppose a girl in your homeroom has been spreading rumors that you asked an ostrich to prom, but you’re so ugly he bit you and then stuck his head in the sand. Nobody really believes the rumors, but people have been avoiding you and giving you strange looks. Publicly saying something like, “It’s okay to be insecure, but I never make fun of your severe acne and massive underbite, so please leave me alone,” will leave the girl feeling humiliated, and give her greater reason to spread ever worse rumors: Maybe a centipede rejected your invite to the Spring Fling in order to make out with a cockroach in the cafeteria.