I think there can be a number reasons.
Some are very insecure people who find it makes them feel better about themselves to hurt others. It makes them feel bigger and stronger to bully those weaker than them.
Some like the power rush over other people, they need to feel like the top person who everyone else must 'bow down to'.
Some enjoy seeing others in pain, they are sadistic in nature, and find it fun to hurt others.
Some bully because they have or are being bullied. Maybe a kid bullies others at school because he or she has problems at home, and it is their way of lashing out because they are unable to do so at home.
Whatever the reasons bullying is wrong and should not be tolerated. Bullies have issues themselves like lack of self confidence and no self esteem