Ancient writers from Aristotle to Cicero to Montaigne described friendship as the pre-eminent human institution. You can go without marriage, or justice, or honor, but friendship is indispensable to life. Each friendship, they continued, has positive social effects. Lovers face each other, but friends stand side-by-side, facing the world — often working on its behalf. Aristotle suggested that friendship is the cornerstone of society. Montaigne thought that it spreads universal warmth.
— David Brooks, in There Are Social and Political Benefits to Having Friends – NYTimes.com.