"PHILOSOPHICALLY SPEAKING...":

Aristotle was overly given to categorizing things, not always correctly, but his observations about friendship have stood the test of time rather well. In the Nichomachean Ethics, Aristotle distinguishes between friends of utility (people who are friends because they are useful to each other), friends of pleasure (people who are friends because they enjoy doing the same things), and friends of virtue (people who are friends because they value the same things, want the best for the other, and respect each other as equals). Because what is useful to us or is "fun" often changes over time, the first two kinds of friendships can be rather short lived, but friends of virtue may endure.

"Now those who love each other for their utility do not love each other for themselves but in virtue of some good which they get from each other. So too with those who love for the sake of pleasure; it is not for their character that men love ready-witted people, but because they find them pleasant. ... And thus these friendships are only incidental; for it is not as being the man he is that the loved person is loved, but as providing some good or pleasure. Such friendships, then, are easily dissolved, if the parties do not remain like themselves; for if the one party is no longer pleasant or useful the other ceases to love him. ... Perfect friendship is the friendship of men who are good, and alike in virtue.... mutual love involves choice and choice springs from a state of character; and men wish well to those whom they love, for their sake, not as a result of feeling but as a result of a state of character. And in loving a friend men love what is good for themselves; for the good man in becoming a friend becomes a good to his friend. Each, then, both loves what is good for himself, and makes an equal return in goodwill and in pleasantness; for friendship is said to be equality, and both of these are found most in the friendship of the good."
classics.mit.edu/Aristotle/nicomachaen.8.viii.html