Why do you think some people bully others?

well,from being bullied severely for about 3 years myself,i know a bit about bullying.A bully only picks on others because of many things.They are jealous,they have problems at home,they want attention but the sad thing is that some bullies got to far,eg.One of my best friends hung himself earlier this year,because he was being bullied and felt like no-one cared for him,he was 13 and had a twin brother.ITS NOT FAIR!!!