There is no one specific reason why people choose to bully others.
Some people bully because they do not understand what they are doing is wrong. Others do it because they're mimicking behaviour they've seen at home.
Many children bully at one time or another as they learn different ways of interacting socially, but stop when it is pointed out to them that they are causing harm.
Often young people have no particular feeling towards those they bully, but use it as a way to get or keep a social position or power within their group. Some people bully to prevent it happening to them.
Some children who have been bullied, go on to bully others. Some children simply have a noticeable lack of empathy or do not accept of value differences.
Anyone can be bullied for any reason. Sometimes it is just a matter of being in the wrong place at the wrong time.
If you're being bullied tell someone about it and remember it's not your fault!
If you're bullying someone, stop - because it hurts people!