Why do people hurt each other?

If only everyone could get along.

That’s impossible. People have emotions.

A troublesome attribute that’s not only invisible, but also change easily.

if it’s friendship or love, then that’s fine but if by some trigger it changes to malice and vengeance, then that could bring forth thoughts of evil.

If parents are fighting all the time, but they still get along well that’s because they love each other.
A person can easily become the giver or the receiver of pain.

In order to keep something like that from happening, one must always hold good feelings for others.