Good question, J! Aristotle says this in Book 9, Chapter 4 of his Nicomachean Ethics (1166a30), where he seems to make true friendship an attribute of the virtuous man exclusively. Just as a man loves his own goodness, so he recognizes and loves that goodness in another, i.e. recognizes a second self in that other. At least that's what I make of the passage; let us know what you think.
It's nice to be discussing real Greek for a change!