Aristotle's account of Friendship is fascinating. abishai is correct about the three types of friendships, which were also hierarchical for Aristotle. Children are friends of pleasure, they love each other as long as they are both happy. As soon as one ceases to be happy, the friendship ends. These are the "weakest" forms of friendship, though they can be the most exciting as well; ain't that typical. The next "level" if you will, is the friendship of use. These are the vast majority of our friendships; more so than many of us would like to admit. These friendships are profitable for "both" parties, and can be long lasting lingering evening after the "use" fads. The last, naturally, is that of "true" friendship, based on virtue as abishai points out. These are rare, and occur ONLY when two virtuous individuals come together in friendship.
This leads to the really crazy claim Aristotle makes. Insofar as the "best for us" would be to become a god, Aristotle argues that a "true" friend would wish their friend to become a god. But here is the rub, the gods do not "need" friends, hence, you are wishing your friend no longer need you; you wish to lose your friend. The essence of virtue according to Aristotle. I have always thought he was onto something . . .