They can then say, as the blessed souls say in Dante, 'Here comes one who will augment our loves.' For in this love 'to divide is not to take away.” Two friends delight to be joined by a third, and three by a fourth, if only the newcomer is qualified to become a real friend.

Hence true Friendship is the least jealous of loves. By myself I am not large enough to call the whole man into activity I want other lights than my own to show all his facets. In each of my friends there is something that only some other friend can fully bring out. But two, far from being the necessary number for Friendship, is not even the best. Above all, Eros (while it lasts) is necessarily between two only. Lovers are normally face to face, absorbed in each other Friends, side by side, absorbed in some common interest. Lovers are always talking to one another about their love Friends hardly ever about their Friendship. The rest of us know that though we can have erotic love and friendship for the same person yet in some ways nothing is less like a Friendship than a love-affair.

“Those who cannot conceive Friendship as a substantive love but only as a disguise or elaboration of Eros betray the fact that they have never had a Friend. Nothing so enriches an erotic love as the discovery that the Beloved can deeply, truly and spontaneously enter into Friendship with the Friends you already had to feel that not only are we two united by erotic love but we three or four or five are all travelers on the same quest, have all a common vision.” But you will have no jealousy at all about sharing the Friendship. If one who was first, in the deep and full sense, your Friend, is then gradually or suddenly revealed as also your lover you will certainly not want to share the Beloved’s erotic love with any third. But this, so far from obliterating the distinction between the two loves, puts it in a clearer light. And conversely, erotic love may lead to Friendship between the lovers. Indeed, unless they are physically repulsive to each other or unless one or both already loves elsewhere, it is almost certain to do so sooner or later. “When the two people who thus discover that they are on the same secret road are of different sexes, the friendship which arises between them will very easily pass – may pass in the first half hour – into erotic love.

The Four Loves by C.S. Lewis