Oliver Wendell Holmes

"Don't flatter yourself that friendship authorizes you to say disagreeable things to your intimates. The nearer you come into relation with a person, the more necessary do tact and courtesy become. Except in cases of necessity, which are rare, leave your friend to learn unpleasant things from his enemies; they are ready enough to tell them." - Oliver Wendell Holmes

(from the perpetual calendar I've had for many years)