“Never give your enemies any more reason than they already have to go on hating you.”

1950s, Loving Your Enemies (November 1957)
Context: There’s another reason why you should love your enemies, and that is because hate distorts the personality of the hater. We usually think of what hate does for the individual hated or the individuals hated or the groups hated. But it is even more tragic, it is even more ruinous and injurious to the individual who hates. You just begin hating somebody, and you will begin to do irrational things. You can’t see straight when you hate. You can’t walk straight when you hate. You can’t stand upright. Your vision is distorted. There is nothing more tragic than to see an individual whose heart is filled with hate. He comes to the point that he becomes a pathological case. For the person who hates, you can stand up and see a person and that person can be beautiful, and you will call them ugly. For the person who hates, the beautiful becomes ugly and the ugly becomes beautiful. For the person who hates, the good becomes bad and the bad becomes good. For the person who hates, the true becomes false and the false becomes true. That’s what hate does. You can’t see right. The symbol of objectivity is lost. Hate destroys the very structure of the personality of the hater. [... ] when you start hating anybody, it destroys the very center of your creative response to life and the universe; so love everybody. Hate at any point is a cancer that gnaws away at the very vital center of your life and your existence. It is like eroding acid that eats away the best and the objective center of your life. So Jesus says love, because hate destroys the hater as well as the hated.

1950s, Loving Your Enemies (November 1957)
Context: Now there is a final reason I think that Jesus says, "Love your enemies." It is this: that love has within it a redemptive power. And there is a power there that eventually transforms individuals. That’s why Jesus says, "Love your enemies." Because if you hate your enemies, you have no way to redeem and to transform your enemies. But if you love your enemies, you will discover that at the very root of love is the power of redemption. You just keep loving people and keep loving them, even though they’re mistreating you. Here’s the person who is a neighbor, and this person is doing something wrong to you and all of that. Just keep being friendly to that person. Keep loving them. Don’t do anything to embarrass them. Just keep loving them, and they can’t stand it too long. Oh, they react in many ways in the beginning. They react with bitterness because they’re mad because you love them like that. They react with guilt feelings, and sometimes they’ll hate you a little more at that transition period, but just keep loving them. And by the power of your love they will break down under the load. That’s love, you see. It is redemptive, and this is why Jesus says love. There’s something about love that builds up and is creative. There is something about hate that tears down and is destructive. So love your enemies.

When asked by an army officer, appointed governor of a west Indies island and who had no experience in law, how to apply the law. Quoted by John Cordy Jeaffreson in A Book About Lawyers http://books.google.com/books?id=lUpqPJSlBS8C&q="tut+man+decide+promptly+but+never+give+any+reasons+for+your+decisions+your+decisions+may+be+right+but+your+reasons+are+sure+to+be+wrong"&pg=PA85#v=onepage, Volume 1 (1867).

Source: The Glass Castle

Enchantment (1999)

Variant: If you've never been hated by your child, you've never been a parent.