Conquer anger by love. Conquer evil by good. Conquer the stingy by giving. Conquer the liar by truth. - Buddhism
Do good to him who has done you an injury. - Taoism
You have heard that it was said, "You shall love your neighbor and hate your enemy." Biut I say to you, love your enemies, pray for them those that persecute you, so that you may be the sons of your Father in Heaven. For He makes the sun rise on the eveil and on the good, and sends rain on the just and on the unjust... You therefore, must be perfect, as your Heveanly Father is perfect. - Christianity
It's easy to be full of love and charity to those that love you. But in striving towards perfection, we must have charity towards those that aren't our friends as well; not just the strangers but also those who do us harm. We aren't required to spend time with them and be their best friend but we are required to treat them as we would treat all human beings, good and bad.