A lot of it is about fitting in. My own son got a rough ride when we moved abroad and he didn't fit in, despite being fluent in the local language. He was different and he really had a rough ride. Some namby pamby child trick cyclist the school insisted he saw tried to tell me that he had low self esteem, which I knew was utter bullshit. I pulled him out of that school there and then despite threats of legal action etc and told them that if I as a parent had allowed my kid to be treated in that way, I'd rightfully be in serious trouble.
After a term at home, he started in a new school and was one of the tougher kids, though he never used this to bully other kids. The experience of being bullied like this was a life lesson for him. A couple of years later some of the kids who had been bullying him saw him around and decided to have a go at him in the street, he beat the crap out of all three of them. He's never had a problem since then.