If it's very painful for you to criticize your friends -- you're safe in doing it. But if you take the slightest pleasure in it, that's the time to hold your tongue.
Source: In The Book of Positive Quotations, by John Cook, 1997.
-- Alice Duer Miller, (Jul 28 1874-1942), US poet, author; She is best remembered for White Cliffs, 1941, a long narrative poem extolling Britain's resistance during WW II; also wrote I Have Loved England.