About the unique friend, and the way he enlarges those souls with whom his soul interacts, C.S. Lewis once famously wrote:
Lamb says somewhere that if, of three friends (A, B, and C), A should die, then B loses not only A but “A’s part in C,” while C loses not only A but “A’s part in B.” In each of my friends there is something that only some other friend can fully bring out. 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. Now that Charles is dead, I shall never again see Ronald’s reaction to a specifically Caroline joke. Far from having more of Ronald, having him “to myself” now that Charles is away, I have less of Ronald. Hence true Friendship is the least jealous of loves. 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. 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.” Of course the scarcity of kindred souls — not to mention practical considerations about the size of rooms and the audiblilty of voices — set limits to the enlargement of the circle; but within those limits we possess each friend not less but more as the number of those with whom we share him increases.