Aristotle, Nicomachean Ethics
H. Rackham, Ed.

[6] The friendships of useful and pleasant people last longer, in fact as long as they give each other pleasure or benefit. It is friendship based on utility that seems most frequently to spring from opposites, for instance a friendship between a poor man and a rich one, or between an ignorant man and a learned; for a person desiring something which he happens to lack will give something else in return for it. One may bring under this class the friendship between a lover and the object of his affections, or between a plain person and a handsome one. This is why lovers sometimes appear ridiculous when they claim that their love should be equally reciprocated; no doubt if they are equally lovable this is a reasonable demand, but it is ridiculous if they have nothing attractive about them.