You have heard that it was said, "You shall love your neighbor and hate your enemy." But I say to you, love your enemies, and pray for those who persecute you, that you may be children of your heavenly Father, for he makes his sun rise on the bad and the good, and causes rain to fall on the just and the unjust.--Matthew 5:43-45
In the world of evolutionary game theory that governs merely natural interactions, altruism is reserved for kith and kin. Just as the always aggressive "hawk" strategy loses out in game theoretical models of the natural world, so too does the altruistic "dove" strategy. The mathematically dominant strategy is most often the reciprocal strategy known as "tit-for-tat." The Church recognizes that a society cannot survive within the natural order of our fallen world without something like a tit-for-tat strategy. This is why Just War Theory allows for self-defense against enemies.
This is appropriate. The Law that God gave to the Israelites on Mount Sinai made similar allowances. But in His definitive revelation of Himself in the Sermon on the Mount and in His non-violent non-resistence to His sacrificial death on Mount Calvary, Our Lord calls us beyond the law that governs beasts in nature and nations at war. The new law is that we are to love our enemies. In the natural order, this is impossible.
It is only possible for perfect people in a perfected world. Our Lord knows this, of course. He concludes today's Gospel by urging us to "be perfect, just as your heavenly Father is perfect." (Matthew 5:48). But how are we to be perfect? In the natural order, survival via the "dove" strategy is impossible. But with the supernatural grace of the Holy Spirit, anything is possible. Even the impossibility of living as a dove.