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.

Holmes cautions against treating intimacy as a license for bluntness. The quote argues that closeness increases, rather than relaxes, the obligation to be tactful: words land with greater force when they come from someone trusted. He distinguishes between necessary candor (rare, and presumably motivated by care or duty) and the common habit of venting irritation or “telling hard truths” under the banner of friendship. The final sentence is deliberately sharp: if a friend must hear something unpleasant, enemies will supply it readily—so a friend should not rush to add to the injury unless doing so serves a genuine moral or practical need.