There is no single reason. A person who bullies may:
Want power, status, attention, or control.
Have learned aggressive behaviour from people around them.
Struggle with empathy or understanding the impact of their actions.
Feel insecure and try to make someone else feel smaller.
Be influenced by a peer group that rewards cruelty.
Be dealing with difficulties in their own life—but this is an explanation, not an excuse.
Target someone because they perceive them as different or vulnerable.
It is important not to assume that every person who bullies has been bullied themselves. Sometimes the simplest explanation is that they have learned that hurting others gets them something they want.