What is "Yoled Kesil l'Sugah Lo"?
R. Yonah: Kesil is one with bad Midos. He causes sadness to his father.
Malbim: Sometimes a Kesil has much Chachmah, just his desire overpowers him blinds his eyes and leads him astray. There is a Kesil from birth - he was born with a nature of great desire. There is a Kesil via habit and learning. "Ka'as l'Aviv Ben Kesil" (25); even so, if his folly is not natural, even though his father does not rejoice in him, he is not filled with sadness. He hopes that he will be trained in a good way and abandon folly. However, if his folly is natural from birth, he is sadness to his father, for there is no hope for him.
Why does a Naval's father not rejoice?
R. Yonah: A Naval is disgraced and lowly. He did not acquire Chachmah and intellectual attributes. It is like "uv'Achariso Yihyeh Naval" (Yirmeyah 17:11) - an expression of descent and disgrace. Kesil is worse than Naval, for Kesil goes in the ways of Iveles, and carries out his bad Midos in action. Therefore, he causes sadness to his father. A father has no contentment and Simchah from his wealth and acquisitions if he knows that what he toiled for will pass to a Naval (disgraced and lowly) son.
Malbim: Naval is the opposite of Nadiv. It is usually via habit. Sometimes a Naval father teaches and habituates his son to be a Naval, and [the son] rejoices is happy, but the father will not rejoice, for he will know that his Simchah is sadness.
