C.S. Lewis outlined Friendship as that love which is not necessary to the biological functions of humanity. Without Eros none of us would have been begotten and without Affection none of us would have been reared; but we can live and breed without Friendship. The former two can be felt tugging at your guts and fluttering in your diaphragm. But in Friendship – in that luminous, tranquil, rational world of relationships freely chosen – you got away from all that. This alone, of all the loves, seemed to raise you to the level of gods or angels. (An example to show the innate difference of perspective here would be that lovers are normally depicted in the mind’s eye as being face to face, absorbed in each other; Friends are side by side, absorbed in some common interest.)
I sometimes wonder if Lewis would say that there is inherent value in Friendship because of (not in spite of) the fact that it is not :necessary” to human existence in the way the other loves are. Being entirely volitional, Friendship can become a thing unto itself, serving those who enter into it either to heaven or hell – which are real but not tangible.