For a practitioner of love and compassion, an enemy is one of the most important teachers. Without an enemy you cannot practice tolerance, and without tolerance you cannon build a sound basis of compassion. So in order to practice compassion, you should have an enemy.When you face your enemy who is going to hurt you, that is the real time to practice tolerance. Therefore, an enemy is the cause of the practice of tolerance; tolerance is the effect or result of an enemy. So those are cause and effect. As is said, Once something has the relationship of arising from that thing, one cannot consider that thing from which it arises as a harmer; rather it assists the production of the effect.
~ Śāntideva
