When we celebrate patriotism on national holidays like the Fourth of July, we don't expect our country to be perfect. And that's okay — we still love our country. But when we sit opposite each other at a Valentine's Day dinner, we feel profoundly uneasy, even threatened, if, just then, not everything feels right.

These are the impossible expectations of romantic love: It must be unconditional, constant and, of course, invariably passionate. Anything else casts doubt on whether the love is genuine. And this needless doubt can paralyze or kill a relationship.

It hasn't always been this way. Before dropping hundreds of dollars on a Valentine's Day date or bemoaning loneliness that night, recall how love was regarded in ancient times — and consider whether some of these older incarnations might be worth reviving.

There is no holiday celebrating friendship, but only since the mid-19th century has romance been elevated above other types of love. For most ancient Greeks, for example, friendship was every bit as passionate and valuable as romantic-sexual love. Aristotle regarded friendship as a lifetime's commitment to mutual welfare, in which two people become "second selves" to each other.

In the Bible, King Saul's son Jonathan loves David, the young warrior who slays Goliath, "as his own soul" and swears eternal friendship with him, while David says their friendship surpasses romantic love. Ruth declares her friendship for her mother-in-law, Naomi, in terms equivalent to a marriage vow: "Where you go I will go, where you lodge I will lodge. . . . Where you die I will die."

Today, friendship has been demoted beneath the ideal of romance, but they should be on an equal footing. We tend to regard our friendships as inferior to our romances in passion, intimacy and depth of commitment. Often they're little more than confessionals in which we seek a sympathetic ear to help us fix — or escape — our romances. When Harry met Sally, they progressed from friends to lovers. And on Facebook we're all "friends" now, further downgrading the meaning of what should be a selective and multifaceted bond.

The idea of human love being unconditional is also a relatively modern invention. Until the 18th century, love had been seen, variously, as conditional on the other person's beauty (Plato), her virtues, (Aristotle), her goodness (Saint Augustine) or her moral authenticity (the Swiss philosopher Jean-Jacques Rousseau). Even Saint Thomas Aquinas, perhaps the greatest of all Christian theologians, said we would have no reason to love God if He weren't good.

The myth that love is unconditional comes from the decline of religion. Christianity, for example, teaches that only God loves unconditionally and that humans, being sinners, need God's grace to get anywhere close to unconditional love. After the 18th-century Enlightenment, the divine ability to love unconditionally got attributed to human beings, while the other half of the story — that we need God's grace for it — was sidelined.

But all human love is conditional. We love others because of something, whether their beauty, goodness or power; because they belong to our families; or because they protect and nurture us. By recognizing that all we have is conditional love, we are less likely to give up on our loved ones as quickly as we often do, less likely to be worried if we occasionally fall in and out of love with them or they with us, and less likely to scare them off by expecting their love to be of superhuman strength.

Another idea about love that has changed over time is that true love must be everlasting. But when love ends, it doesn't mean it wasn't true. It's usually replaced with companionship, habit or benevolence rather than enmity. The euphemism that gets tossed around is that we're coming to love someone "in different ways." Often, though, this isn't accurate: We are, in fact, ceasing to love them.

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.

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Simon May, the author of "Love: A History," is a visiting professor of philosophy at King's College London.