What is love without fidelity? In the ultimate analysis, it is nothing but a lie. For the deepest meaning of ever love, the inner “word” uttered in love, is the orientation toward and giving of oneself to the beloved, a giving that knows no time limit. No fluctuation in the course of life can shatter it. Only a deep change in the beloved person can affect our love if it be true love. A man who would say, “I love you now, but how long it will last, I cannot tell,” does not truly love; he does not even suspect the very nature of love. Faithfulness is so essentially one with love, that everyone, at least as long as he loves, must consider his devotion an undying devotion. This holds good for every love, for parental and filial love, for friendship and for spousal love. The deeper a love, the more it is pervaded by fidelity.
~ Dietrich von Hildebrand, The Art of Living
Locus Communis 