Truth
Best friends are like diamonds, precious and rare. False friends are like leaves, found everywhere. (Origin uncertain)
The biblical King David complained about the betrayal of a close friend. That really hurts, doesn’t it? True friends will never betray you, but they may confront you with truth that hurts. David had another friend who faced him directly with his failures (see 2 Samuel 12), and the king repented and served his nation well, thanks to the rebuke of a true friend.
But correct the wise, and they will love you. . . . Open rebuke is better than hidden love (Proverbs 9:8; 27:5).