Anger

  "A passionate temper renders a man unfit for advice, deprives him of his reason, robs him of all that is either great or noble in his nature; it makes him unfit for conversation, destroys friendship, changes justice into cruelty, and turns all order into confusion. Says Lord Bacon: 'An angry man who suppresses his passions, thinks worse than he speaks; and an angry man that will chide, speaks worse than he thinks. A wise man hath no more anger than is necessary to show that he can apprehend the first wrong, nor any more revenge than justly to prevent a second.'

Don't you wish we didn't continue to need to be reminded?
"A True Friend," and photo from the internet.