I think the first reason that we should love our enemies, and I think this was at the very center of Jesus' thinking, is this: that hate for hate only intensifies the existence of hate and evil in the universe.
We are often infinitely mistaken, and take the falsest measures, when we envy the happiness of rich and great men; we know not the inward canker that eats out all their joy and delight, and makes them really much more miserable than ourselves.