Aristotle, Nicomachean Ethics
H. Rackham, Ed.

Also it is by rendering affection in proportion to desert that friends who are not equals may approach most nearly to true friendship, since this will make them equal. Amity consists in equality and similarity, especially the similarity of those who are alike in virtue; for being true to themselves, these also remain true to one another, and neither request nor render services that are morally degrading. Indeed they may be said actually to restrain each other from evil: since good men neither err themselves nor permit their friends to err. Bad men on the other hand have no constancy in friendship, for they do not even remain true to their own characters; but they can be friends for a short time, while they take pleasure in each other's wickedness. [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. [7]

But perhaps there is no real attraction between opposites