Many of the great thinkers of love acknowledged its mortality. Aristotle said that love between two people should end if they are no longer alike in their virtues. Even Jesus seemed to suggest that God's love for humanity isn't necessarily eternal. After all, at the Last Judgment, the righteous will be rewarded with the Kingdom of God — with everlasting love — but those who did not act well in their lives will hear the heavenly judge say: "You that are accursed, depart from me into the eternal fire prepared for the devil and his angels." And Jesus adds: "These will go away into eternal punishment."
Love, in other words, is not as patient, kind or enduring as we might like to think.
And finally, let's release romantic and marital love from the stranglehold of sexual expectation. Sure, sex is an unsurpassed pleasure — but you can have a tremendous erotic bond with a person and have sex only infrequently. The ethos of courtly love in the 12th and 13th centuries — the love of the troubadours — involved intense eroticism but little if any consummation. I'm not suggesting that we revive medieval courtship, but we should think of sex as just one of the bonds and delights of erotic love, rather than as its touchstone. If sex isn't going so well, or if desire is no longer so urgent, this doesn't necessarily mean that we love less urgently, let alone that it's time for a change.
The point of recalibrating our expectations isn't to downgrade romantic love but to make it more successful. We are putting romantic love on a firmer footing if we accept that friendship should play an equal part in meeting our need to love and be loved, that love is much more than romance, that romance needn't live or die by sex, and that because love is conditional, we needn't always worry if it wavers. To believe, on the contrary, that our love is true only if it is unconditional and unchanging is to play God — and that always ends badly.
Simon May, the author of "Love: A History," is a visiting professor of philosophy at King's College London.