5:15-17 — 15 Drink water from your own cistern, flowing water from your own well. 16 Should your springs be scattered abroad, streams of water in the streets? 17 Let them be for yourself alone, and not for strangers with you.
Though erotic images may be suggested here (ANE texts suggest this, cf. Longman), there is also the sensible aspect to these verses. Contrasted to the life of waste in vv. 9-11, we are called to remain at home. Don’t waste your energy on some strange woman; drink from your own well (cf. Song of Sol. 4:10-15)— be homebound, not a carouser. Namely, we should be content with our own spouses and not go beyond.
The man who often travels away from home for business or other excursions (hobbies, interests, etc.) will not be able to drink from his own cistern. The practical effect is to open himself up to temptations or these extra curricular activities are in fact conscious willful defections away from his wife. None of us should presume that we are islands to ourselves; go far from your own cistern and you will inevitably fall into temptation. Admittedly, a few exceptions to the rule may exist but they are exceptions and not the rule.
5:18-20 — 18 Let your fountain be blessed, and rejoice in the wife of your youth, 19 a lovely deer, a graceful doe. Let her breasts fill you at all times with delight; be intoxicated always in her love. 20 Why should you be intoxicated, my son, with a forbidden woman and embrace the bosom of an adulteress?
A man is to be content with his own wife. “But it is highly important to see sexual delight in marriage as God-given; and history confirms that when marriage is viewed chiefly as a business arrangement, not only is God’s bounty misunderstood, but human passion seeks (cf. verse 20) other outlets.” (Kidner)
Sexual contact and satisfaction are limited only to married couples. Adultery and fornication will lead us to death. Why should a man go somewhere else when he has been provided for? Why drink stolen water when you have your own supply? The language of intoxication suggests that satisfaction can occur in the embrace of one’s own wife or the in embrace of another woman. Theses commands “rejoice in the wife of your youth” and “let her breasts fill you at all times with delight” imply conscious effort — it just does not happen without prayerful commitment and conscious pursuit. This intoxication erupts with willful decisions; it does not fall from the sky. The silly statement, “We fell out of love” only means “We stopped trying.” God commands the man to rejoice in his wife and be satisfied with her. This also means that the wife has her godly role in fostering and enabling this to happen. It take two to tango, does it not?
5:21-23 — 21 For a man’s ways are before the eyes of the LORD, and he ponders all his paths. 22 The iniquities of the wicked ensnare him, and he is held fast in the cords of his sin. 23 He dies for lack of discipline, and because of his great folly he is led astray.
With these solemn warnings, the father reminds the son that God is well aware of each man’s ways. His sins will trip him up and in the end they will destroy him. “In conclusion the father grounds his teaching in theology, that is, the LORD’s omniscience (v. 21) and justice (vv. 22-23).” (Waltke) Also note, “The linkage assumes that sin against God and insolence toward the wisdom instruction are inseparable.” (Waltke)
God is watching, and so the punishments of vv. 22-23 (ultimately death) are not a matter of chance, but certainty; the implication is that no matter what particular form the punishment might take, God will assure that it will happen. The sin of the adulterers will come back and harm them (v. 22). If they are not inebriated by the love of their wife, then they will be inebriated by their own stupidity, and that will result in their death. (Longman)