Those only are happy who have their minds fixed on some object other than their own happiness: on the happiness of others, on the improvement of mankind, even on some art or pursuit followed not as a means, but as itself an ideal end. Aiming at something else, they find happiness by the way.
When we hate our enemies, we are giving them power over us: power over our sleep, our appetites, our blood pressure, our health, and our happiness. Our enemies would dance with joy if only they knew how they were worrying us, lacerating us, and getting even with us! Our hate is not hurting them at all, but our hate is turning our own days and nights into a hellish turmoil.