In meditation, imagine that in front of you are three persons—an enemy, a friend, and a neutral person. At that time, in our minds we have (1) a sense of closeness for one of them, thinking, “This is my friend”; (2) a sense of dislike even when imagining the enemy; and (3) a sense of ignoring the neutral person. Now, we have to think about the reasons why we generate these feelings—the reasons being that temporarily one of them helped us whereas the other temporarily harmed us, and the third did neither. However, when we think in terms of the long course of beginningless rebirth, none of us could decide that someone who has helped or harmed us in this life has been doing so for all lifetimes.

When you contemplate this way, eventually you arrive at a point where a strong generation of desire or hatred appears to you to be just senseless. Gradually, such a bias weakens, and you decide that one-sided classification of persons as friends and enemies has been a mistake. ~  His Holiness The 14th Dalai Lama, The Dalai Lama at Harvard: Lectures on the Buddhist Path to Peace, page 166
page 166.