Some great CS Lewis quotes from the chapter on friendship in his book, 'The Four Loves':
Friendship is - in a sense not at all derogatory to it - the least natural of loves; the least instinctive, organic, biological, gregarious and necessary. It has least commerce with our nerves; there is nothing throaty about it... 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 species, biologically considered, has no need of it.
Lovers are always talking to one another about their love; Friends hardly ever about their Friendship. Lovers are normall face to face, absorbed in each other; Friends, side by side, absorbed in some common interest.
In each of my friends there is something that only some other friends 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.
Friendship arises out of mere Companionship when two or more of the companions discover that they have in common some insight or interest of even taste which the others do not share and which, till that moment, each believed to be his own unique treasure (or burden). the typical expression of opening Friendship would be something like, "What? You too? I thought I was the only one."
...In this kind of love [Friendship]... Do you love me? means Do you see the same truth? - Or at least, "Do you care about the same truth?" The man who agrees with us that some question, little regarded by others, is of great importance, can be our Friend.
...In Friendship... we think we have chosen our peers. In reality, a few years' difference in the dates of our birth, a few more miles between certain houses, the choice of one university instead of another, posting to different regiments, the accident of a topic being raised or not raised at a first meeting - any of these chances might have kept us apart. But, for a Christian, there are, strictly speaking, no chances. A secret Master of the Ceremonies has been at work. Christ, who said to the disciples "Ye have not chosen me, but I have chosen you," can truly say to every group of Christian friends "You have not chosen one another, but I have chosen you for one another."