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.