Let thy fountain be blessed - יהי מקורך ברוך yehi mekorecha baruch. Sit vena tua benedicta. Thy vein; that which carries off streams from the fountain of animal life, in order to disperse them abroad, and through the streets. How delicate and correct is the allusion here! But anatomical allusions must not be pressed into detail in a commentary on Scripture.

Let thy (k) fountain be blessed: and rejoice with the wife of thy (l) youth.
(k) Your children who will come from you in great abundance showing that God blesses marriage and curses whoredom.
(l) Who you married in your youth.

Let thy fountain be blessed,.... Thy wife; make her happy by keeping to her and from others; by behaving in a loving, affable, and respectful manner to her; by living comfortably with her, and providing well for her and her children: or reckon her a happiness, a blessing that God has bestowed; or
"thy fountain shall be blessed,''
as the Targum; that is, with a numerous offspring, which was always reckoned a blessedness, and was generally the happiness of virtuous women, when harlots were barren;
and rejoice with the wife of thy youth; taken to be a wife in youth, and lived with ever since; do not despise her, nor divorce her, even in old age, but delight in her company now as ever; carry it not morosely and churlishly to her, but express a joy and pleasure in her; see Ecclesiastes 9:9. Jarchi interprets this of the law learned in youth; but it might be much better interpreted of the pure apostolic church of Christ, "the beulah", to whom her sons are married, Isaiah 62:4; to whom they should cleave with delight and pleasure, and not follow the antichristian harlot.

wife . . . youth--married in youth.

With Proverbs 5:18 is introduced anew the praise of conjugal love. These three verses, Proverbs 5:18-21, have the same course of thought as Proverbs 5:15-17.
18 Let thy fountain be blessed,
And rejoice in the wife of thy youth.
19 The lovely hind and the graceful gazelle -
May her bosom always charm thee;
In her love mayest thou delight thyself evermore.
20 But why wilt thou be fascinated with a stranger,
And embrace the bosom of a foreign woman?
Like בור and באר, מקור is also a figure of the wife; the root-word is קוּר, from קר, כר, the meanings of which, to dig and make round, come together in the primary conception of the round digging out or boring out, not קוּר = קרר, the Hiph. of which means (Jeremiah 6:7) to well out cold (water). It is the fountain of the birth that is meant (cf. מקור of the female ערוה, e.g., Leviticus 20:18), not the procreation (lxx, ἡ σὴ φλέψ, viz., φλὲψ γονίμη); the blessing wished for by him is the blessing of children, which בּרוּך so much the more distinctly denotes if בּרך, Arab. barak, means to spread out, and בּרך thus to cause a spreading out. The מן, 18b, explains itself from the idea of drawing (water), given with the figure of a fountain; the word בּאשׁת found in certain codices is, on the contrary, prosaic (Fl.). Whilst שׂמח מן is found elsewhere (Ecclesiastes 2:20; 2-Chronicles 20:27) as meaning almost the same as שׂמח בּ; the former means rejoicing from some place, the latter in something. In the genitive connection, "wife of thy youth" (cf. Proverbs 2:17), both of these significations lie: thy youthful wife, and she who was chosen by thee in thy youth, according as we refer the suffix to the whole idea or only to the second member of the chain of words.

Fountain - Thy wife. Blessed - With children; for barrenness was esteemed a curse among the Israelites.