C. F. Keil and Franz Delitzsch on Proverbs 27:2

Pro 27:2 2 Let another praise thee, and not thine own mouth;    A stranger, and not thine own lips.

The negative לא is with פיך, as in (Arab.) ghyra fyk, bound into one compact idea: that which is not thine own mouth (Fleischer), “not thine own lips,” on the other hand, is not to be interpreted as corresponding to it, like אל־מות, Pro 12:28; since after the prohibitive אל, יהללוּך [praise thee] easily supplies itself. זר is properly the stranger, as having come from a distance, and נכרי he who comes from an unknown country, and is himself unknown (vid., under Pro 26:24); the idea of both words, however, passes from advena and alienigena to alius. There is certainly in rare cases a praising of oneself, which is authorized because it is demanded (2Co 11:18), which, because it is offered strongly against one’s will, will be measured by truth (Pro 10:13); but in general it is improper to applaud oneself, because it is a vain looking at oneself in a glass; it is indecent, because it places others in the shade; imprudent, because it is of no use to us, but only injures, for propria laus sordet, and as Stobäus says, οὐδὲν οὕτως ἄκουσμα φορτικὸν ὡς καθ ̓ αὑτοῦ ἔταινος. Compare the German proverb, “Eigenlob stinkt, Freundes Lob hinkt, fremdes Lob klingt” [= self-praise stinks, a friend’s praise is lame, a stranger’s praise sounds].