It is natural for parents to look to their children for a sense of fulfillment and meaning in their own lives. The possibility of finding joy and gratification in the accomplishments of children helps motivate Jewish parents to make the sacrifices that are needed to raise a new generation that can effectively carry on the Torah way of life.
But a child who turns out badly is bound to cause such anguish to his parents as to banish almost every other joy from their lives. Mishlei urges parents to do their utmost to secure happiness, and not sorrow, from their children.
כא = יֹלֵד כְּסִיל לְתוּגָה לוֹ וְלֹא יִשְׂמַח אֲבִי נָבָל
(21) The man who fathers a fool has done so to his sorrow,
and the father of a disreputable son will not rejoice.
This proverb looks at what might make a father feel a sense of disappointment in his son. Two types of disappointment are identified, one in which the father is immersed in sadness and the other in which the father is not actually upset, but he misses the sense of joy that he would like to have had.
Of course, the qualities that will have these kinds of effects depend upon what kind of person the father himself is and upon what his expectations are.
Therefore, Mishlei uses vague descriptive terms that can be interpreted in various ways. The son that brings sorrow to his father is described as a כְּסִיל. This is a word that is often translated as “fool,” but the qualities that make a person a fool could be almost anything depending on the circumstance. Rabbeinu Yonah Gerondi who lived in Spain about 800 years ago spoke of a כְּסִיל as a person of poor character traits. A person who lacks humility and integrity is likely to live a sinful life, which will cause great sorrow to the parents who raised him.
The son that denies his father the possibility of rejoicing is referred to as a נָבָל or disreputable person. Rabbeinu Yonah characterizes him as being one who has not developed his intellectual faculties. He may not be a sinner, but he will never become a Torah scholar. Therefore, he does not bring his father joy.
(21) He who fathers a fool — יֹלֵד כְּסִיל
has done so to his sorrow — לְתוּגָה לוֹ
nor will he rejoice — וְלֹא יִשְׂמַח
as the father of a disreputable son — אֲבִי נָבָל.
 The word כְּסִיל may be understood as an intelligent person who can distinguish right from wrong, but his character is weak and so he does whatever is expedient because he gives in to his passions. (מלבי”ם)
 The word נָבָל may be understood as a person who engages in crude or inappropriate speech. (אבן עזרא)
 The word נָבָל may be also understood as referring to a stingy person. Even if his father has the same character defect (and even if he trained his son to be stingy), he will eventually be disappointed by his son’s attitude. (מלבי”ם)
 The proverb may also be speaking of a teacher who sees his student as a potential source of joy and fulfillment. The teacher is disappointed when the efforts he put into the student fail to bear fruit. (הגר”א)
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Mishlei 17-21 (Parenting) PDF version