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. 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. And conversely, erotic love may lead to Friendship between the lovers. But this, so far from obliterating the distinction between the two loves, puts it in a clearer light. 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 you will have no jealousy at all about sharing the Friendship. 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.
C.S. Lewis, The Four Loves
friendship