Why does child abuse occur?

Some parents abuse their kids because they have an alcohol or drugs problem, or they have an extreme temper and they take it out on their kids, some parents abuse their kids because the parents went through something traumatic like losing a wife or husband (but that's still not an excuse to start abusing your child!) Some parents abuse their children because they were abused when they were children, and then, you get some people who are just plain cruel and enjoy abusing children. Sometimes kids are abused because of temper tantrums or how they act all the time to their parents.