Because young children might think that it is a natural way to treat other people based on how they've been treated.

Bullying often stems from either insecurity, or conversely from a person being so secure that they become indifferent to other peoples feelings.

They don't listen to Katy Perry.

I think the main reason is to fit in with others, or to look cool. Sometimes, bullies feel insecure or depressed, and pick on others to make them feel better about themselves.

Insecurity
Self Loathing
Jealousy
Wanting to fit in
Hiding something
They're just a bitch plain and simple

There are lots of different factors, but I think that it all comes down to a desire for power. Bullies want to feel like they're in control of their lives, and they want to feel like they have authority. So they exert what little authority they have over the only people who will submit to them; namely, their victims.

Why do you think mall cops and security guards are always harassing customers over petty, inconsequential crap? Because they want to feel powerful and important, even though they know deep down that they're not.

Same, in my experience bullies do it to look cool in front of their friends. And then when they're by themselves it's a completely different story

Bullied children are usually bullied at home. Bully's arent popular :P they're sad and lonely.
Like most of the people on this site.