From a moral point of view it is, of course, a duty for one of the friends to point out the other's faults to him; this is in the other's best interests and is therefore a duty of love. But the latter sees in this a lack of the respect he expected from his friend and thinks that he has either already lost or is in constant danger of losing something of his friend's respect, since he is observed and secretly criticized by him; and even the fact that his friend observes him and finds fault with him will seem in itself offensive.
(Immanuel Kant, The Metaphysics of Morals, trans. and ed. Mary Gregor, Cambridge Texts in the History of Philosophy, ed. Karl Ameriks and Desmond M. Clarke [Cambridge: Cambridge University Press, 1996 (first published in 1797)], 215-6)
Note from KBJ: With friends like this, who needs enemies?
