Why do kids bully?

My name is Yao Yao.I’m a bullied kid.However,I wasn’t bullied very often,so I had a happy life.But,I didn’t forget who bullied me.
This kid in my class bullied me.I told my teacher.So he stopped bullying me.He never bullied me again.
Another kid who bullied me was this girl. She was in my class,too.She once laughed at how I dribbled a ball.I didn’t tell my teacher,since she didn’t bully me so often.
This last kid who bullied me was a kid.I’m in 4th grade,and he’s in 5th grade.One day I made the mistake of staying on the back.In my school,when you go on a bus,3rd graders are in the front,4th graders in the middle,and 5th graders in the back in a whole line.I stayed far back,and then he started bullying me."You’re so ugly," he said.
Well,I screamed.However,that didm’t stop him.
"Your hair is so ugly," he said.I was reading a book."You’re so dumb because you love reading,and reading makes you dumb."
I ignored him.