In all friendships between dissimilar people, as we have said, it is proportion that produces equality and preserves the friendship; for example, in political friendship the shoemaker receives in return for his shoes what they are worth, and so do the weaver and the rest. In these cases, a common measure is provided in the form of money, and so everything is referred to this and measured by it.
But in erotic friendship the lover sometimes complains that his profound love is not requited (perhaps because he has no quality worthy of love), while the beloved often complains that the lover formerly promised him everything and now does nothing.
Things like this happen when the lover loves the beloved for pleasure, while the beloved loves the lover for utility, and these objects are no longer obtained by both of them. For if these are the reasons for the friendship, it is dissolved when they do not get what they were aiming at in their love; each was fond not of the other himself, but of his qualities, and since these do not last, such friendships do not last either. But friendship of character, since it exists for its own sake, does last, as we have said.
People quarrel when what they get is different from what they want, because not getting what one aims at is like getting nothing at all.