The quote that summarizes the whole book for me was this:
Man can ascend to Heaven only because the Christ, who died and ascended to Heaven, is ‘formed in him.’ Must we not suppose that the same is true of man’s loves? Only those into which Love Himself has entered will ascend to Love Himself.
Lewis places humanity in dependence on God. Salvation and eternity hang in the balance apart from God's intervention. Similarly, the ability to love hangs in the balance apart from God's intervention.
Certainly, humanity is capable of a form of love, just as a toddler does a form of walking. Personally, I've been known to do a form of singing. But it's not quite the real thing. Outside influence, further development and refining are all necessary to go from the form to the reality. Lewis says this comes from God because God is love. His reasoning could easily be based on the New Testament passage 1 John 4:7-12.
This sort of Divine-influenced, divine-like love must be king in a person's heart before they can navigate the other "loves" that stir in their lives. Whether it's a love for chess, white mice, a blonde beauty, or an old book, all such loves need to be ordered under the King of Love--Charity herself, the love marked by personal sacrifice and goodness for another, most of all goodness toward God.
With Charity-love (A.K.A. God-like love) at the helm, we can now love chess without it destroying the lives around us. We can keep mice and go on a date with the blonde and curl up with that book. None of them will take center place, nor will we abuse either gift for our own advantage. We neither make those things gods nor treat them as if we're gods.
So for instance, romance, or erotic love, (what Lewis refers to as Eros), can be a very demanding, god-like love. It moves us to make promises, overhaul convictions, and express things beyond human ability. Or as Lewis explains, “Eros is driven to promise what Eros himself cannot perform.”
We've all read of or seen people "in love" with things they ought not (the neighbor's wife, the bad-boy hooligan, and the like). What would ever break this erotic love? Answer: a stronger love, a king love, a chief love, divine love. Then and only then can Eros be deposed. Then and only then can we follow Lewis' counsel: “Eros driven to a forbidden object, may have to be sacrificed [in light of Charity/Divine Love].”
Pastor John Piper describes our lives as a solar system. With God and His Love at the center of the solar system, the planets (loves) of our lives can be properly ordered. But if something else seeks to control the system, chaos and disorder erupts. This is the lot of natural humanity. We're born in the chaos of disordered loves and disordered lives. Until we see Jesus' Christ death for sinners as the focal point of history and the most grounding reality of the universe and our only hope of salvation, we remain hopeless in a universe of disordered loves.