I Don’t Understand

I don't understand why people are this way.

They can be

Kind, caring, helpful, intelligent, funny, dedicated, loyal, trustworthy, loving.

At the same time, they can also be

Awful, rude, unfaithful, hurtful, uncommitted, arrogant, irrational, hateful.

I can say that I know both,

But still do not understand what makes them that way.

What makes them terrible?

What makes them amazing?

This, I do not understand.