Bullies can have issues with their families; the parents are a common cause. They could have a lack of involvement or supervision towards their kids, or they could be strict, restrictive or have harsh disciplines. Another family reason is victimisation by older siblings.
A very common source of bullying comes from the peers. Friends that bully could look up to other bullies, bullying can be done to enhance or maintain their social status, or those of a low social status could deflect the teasing by bullying others.
Bullying thrives where schools don't address the problem or have a policy (against bullying); this gives students an oppurtunity to attack, especially at lunch, recess or before or after school. Bullying role models in the media can encourage people to be like them, and social aggression is also a cause, as bystanders want action, attention, security or friendships.