Matthew Henry Presbyterian @wholebiblehenry

This intimates both the pleasure and the advantage of conversation. One man is nobody; nor will poring upon a book in a corner accomplish a man as the reading and studying of men will. Wise and profitable discourse sharpens men's wits; and those that have ever so much knowledge may by conference have something added to them.

John Gill Reformed Baptist @doctorgill

As in water face answereth to face,.... As water is as a looking glass, in which a man may behold his own face and another's; or as the face in the water answers to the face of a man, and there is a great likeness between them.

Jamieson-Fausset-Brown Reformed @jfbcommentary

a man sharpeneth . . . friend--that is, conversation promotes intelligence, which the face exhibits.