Picture somebody that you dislike, somebody that you hate, and imagine all of the things that you hate about them: the way they talk, the things they have done to you. Now, as you hold that picture, go back in time and imagine them as a child. See them alone and bullied at school. See the teachers who never bothered to look at them or offer them a kind word. See the parents who always put them down, swore at them, and hurt them. How easy is it still to hate that person?
The American poet Henry Wadsworth Longfellow once wrote that:
"If we could read the secret history of our enemies, we should find in each man's life sorrow and suffering enough to disarm all hostilities."
In his book The Theory of Moral Sentiments, Adam Smith argued that the foundation of all morality is based upon a single act of imagination. It's when we imagine life as somebody else, seeing the world as they see it. Smith called this 'sympathy,' but the word goes beyond how it's commonly used today.
For Smith, sympathy is a deliberate reconstruction of somebody else's inner life. It's when we see the fears, the disappointments, and the thousand humiliations that built the person standing in front of us.
This doesn't mean you need to forgive every wrongdoing. It doesn't mean you need to let that person back into your life. It just means that hate is a kind of chain, and it's a chain that can only be broken when we decide that we won't pass it on anymore.