"Common friendships can be shared. In one friend one can love beauty; in another, affability; in another, generosity; in another, a fatherly affection; in another, a brotherly one; and so on. But in this friendship love takes possession of the soul and reigns there with full sway: this cannot possibly be duplicated. If two friends asked you to help them at the same time, which of them would you dash to? If they asked for conflicting favours who would have the priority?"
-Michel de Montaigne, The Essays of Montaigne XXVII: Of Friendship (1580).