He that walketh with wise men shall be wise - To walk with a person implies love and attachment; and it is impossible not to imitate those we love. So we say, "Show me his company, and I'll tell you the man." Let me know the company he keeps, and I shall easily guess his moral character.

He that walketh with wise [men] shall be wise: but a companion of fools shall be (i) destroyed.
(i) As he is partaker of their wickedness, and bears with their vices, so will he be punished alike as they are.

He that walketh with wise men shall be wise,.... Who is a companion of them that fear the Lord; converses frequently with them in private about spiritual and experimental things, and walks with them in public in all the commandments and ordinances of the Lord; he by those means grows wiser and wiser, gains a large stock of spiritual knowledge and experience; for this holds good both in natural and spiritual wisdom, a man of any capacity at all will improve by keeping wise company;
but a companion of fools shall be destroyed; the Vulgate Latin version renders it, "shall become like them"; be a fool as they are, and grow still more and more foolish. The Septuagint version is, "shall be known"; known by the company he keeps to be a fool also: or rather, "shall be broken" (t); ruined and destroyed, "evil communications corrupt good manners", 1-Corinthians 15:33, and so bring to ruin and destruction.
(t) "conteretur", Pagninus, Montanus, Piscator, Mercerus, Gejerus; "conquassabitur", Cocceius; "fragetur", Michaelis; "infringetur", Schultens, so Ben Melech.

Multitudes are brought to ruin by bad company. And all that make themselves wicked will be destroyed.

The benefits of good and evil of bad society are contrasted.

20 Whoever goes with wise men, becomes wise;
And whoever has intercourse with fools, becomes base.
Regarding the significance of this proverb in the history of the religion and worship of Israel, vid., p. 39. We have translated 20a after the Kerı̂; the translation according to the Chethı̂b is: "go with wise men and become wise" (cf. Proverbs 8:33), not הלוך, for the connection of the (meant imperatively) infin. absol. with an imper. (meant conclusively) is not tenable; but הלוך is an imper. form established by הלכוּ, Jeremiah 51:50 (cf. הלוך = לכת, Numbers 22:14), and appears to have been used with such shades of conception as here as intercourse and companionship for לך. Regarding ירוע gnid, vid., at Proverbs 11:15; there it meant malo afficietur, here it means malus (pejor) fiet. The Venet. (contrary to Kimchi, who explains by frangetur) rightly has κακωθήσεται. There is here a play upon words; רעה means to tend (a flock), also in general to be considerate about anything (Proverbs 15:14; 44:20), to take care of anything with the accusative of the person (Proverbs 28:7; Proverbs 29:3), to hold intercourse with any one: he who by preference seeks the society of fools, himself becomes such (Jerome, similis efficietur), or rather, as ירוע expresses, he comes always morally lower down. "A wicked companion leads his associate into hell."