Full Idea
Friendships are most commonly found in democracies, because the citizens, being equal, have much in common.
Gist of Idea
Democracy is the best constitution for friendship, because it encourages equality
Source
Aristotle (Nicomachean Ethics [c.334 BCE], 1161b09)
Book Reference
Aristotle: 'Ethics (Nicomachean)', ed/tr. ThomsonJ A K/TredennickH [Penguin 1976], p.278
A Reaction
He also implies that friendship promotes democracy, presumably because friends prefer to be equals.