Bullying stems from the idea that everyone has to be “perfect”. “Perfect” being the best possible combination of passiveness and assertiveness, plenty of talents, and a gorgeous face and body. If you don't fit this ideal, you are penalized. People can be teased, excluded, or even “befriended” and treated as a pet that people make fun of. People can be judged from the first time they encounter someone. If their looks aren't perfect (skinny, well-proportioned), they are forever labeled according to their imperfections.
A lot of times, people base their ideas of beauty and character on an enhanced version of themselves. They imagine what they would be like if they were “perfect”, and use that as an expectation for all other people. For example, if Person A has blonde hair, they find redheads to be less than perfect off the bat. Now, Person A might acknowledge that their hair isn’t very soft or their waist not incredibly small, but the basics of their own appearance are how they judge the people they meet.
Many people are bullied because they are far outside the realm of “perfection”. Looks, intelligence, ideals, character, and ambitions are some generalized categories that people use to judge others. In this sense, anyone in any way imperfect is subject to bullying. This is probably why many people consider bullying to be a nationwide epidemic. It can cause depression, eating disorders, low self esteem, and even suicide.
For the most part, everyone is bullied at one point or another. Nobody's “perfect” and therefore nobody's immune to social harassment. Sometimes a person who is especially passive is picked on and taken advantage of. Snide remarks can be made about someone who's slightly overweight in the locker room. People who are too brainy can be excluded from birthday parties and get-togethers. Bullies can be people especially close to the ideal look or personality or they can be people far from it who are merely embarrassed and self-conscious of themselves. A bully can come from as many places as a victim can.
Perfection isn't possible, so criticizing people for falling short of unrealistic expectations is unfair. The true instigator of bullying is the quest for perfection inside and out. People may never fully except that their version of perfect can't be imposed on people, but finding the reasons behind bullying makes it easier to fix the problems in today's society.