It is in the friendship of good men that feelings of affection and friendship exist in their highest and best form. -- Aristotle in Nicomachean Ethics.
I had occasion recently to reflect upon what I thought to be the ingredients of friendship. I ventured that friendship requires that two people share an affection for each other based upon similar values and interests, mutual esteem, and shared experience. To the extent that two persons hold common intellectual, moral, and spiritual values and are bonded by similar personality traits and shared experiences in life, to that extent their friendship will be deep. The less they share in common, either in terms of values, personality, or experience, the more superficial will be any relationship between them.
Atleast that's what I think. There are many other opinions on the matter much more profound and compelling than mine. Of those that I'm aware of, the loveliest expression of the nature of friendship is found in Augustine's Confessions. Augustine, describing his grief at the death of a dear companion, writes of his friends that:
All kinds of things rejoiced my soul in their company - to talk and laugh and do each other kindnesses; read pleasant books together, pass from lightest jesting to talk of the deepest things and back again; differ without rancour, as a man might differ with himself, and when most rarely when dissension arose find our normal agreement all the sweeter for it; teach each other or learn from each other; be impatient for the return of the absent, and welcome them with joy on their home-coming; these and such like things, proceeding from our hearts as we gave affection and received it back, and shown by face, by voice, by the eyes, and a thousand other pleasing ways, kindled a flame which fused our very souls and of many made us one.
This is what men value in friends, and value so much that our conscience judges them guilty if they do not meet friendship with friendship, expecting nothing from their friend save such evidences of his affection. -- Confessions Book IV: viii, ix
Beautifully put.