Albert Barnes Commentary Proverbs 27:17

"Iron sharpeneth iron; So a man sharpeneth the countenance of his friend." — Proverbs 27:17 (ASV)

The proverb expresses the gain of mutual counsel as found in clear, well-defined thoughts. Two minds, thus acting on each other, become more acute. This is better than to interpret “sharpening” as provoking, and to take the maxim's point to be that the quarrels of those who have been friends are bitter in proportion to their previous intimacy.