Lewis on friendship

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... 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.... we possess each friend not less but more as the number of those with whom we share him increases. - - C. S. Lewis, The Four Loves