R. Yonah: If you see someone stumble in his deeds, for he was drawn after his desires, and it will be good in his eyes when they rebuke him, know that he loves Da'as. His Nefesh accepts words of Sechel and chooses Da'as, just desire overpowers him to make him sin. Therefore, amidst love of Da'as he loves Musar, and it is good for him when they rebuke him. He hopes that rebuke will help him to rule over his Ru'ach and overpower his Yetzer.

Malbim: Tochachah is debate with intellectual proofs, showing what is good and what is evil. Musar forces him to accept his words via afflictions, or he fears lest he punish, get angry or command him, like a minor fears an adult. Or, he rebukes and informs Hashem's punishments to those who rebel against His words. One who loves Tochachah surely loves Da'as, for Tochachah is built on proofs of Sechel and Da'as. Our verse teaches that also one who loves Musar loves Da'as, for also the roots of Musar are built on Yir'as Hashem and knowledge of His Hashgachah, reward and punishment, which are explained via Da'as. Da'as is mostly built on what is known via senses, and matters known via senses that are famous and accepted, e.g. what happened to Dor ha'Mabul, Dor ha'Flagah, Sedom, Egypt and Korach's congregation. If so, he loves Da'as.

R. Yonah: Since he hates rebuke, he is a Kesil and a Ba'ar. His Nefesh is drawn after desire; it is like the Nefesh of an animal. The Targum of Behemah is Be'ira. Also "v'Shilach Es Be'iro" (Shemos 22:4, refers to an animal). We find [that Ba'ar is a fool -] "Kesil va'Va'ar Yovedu" (Tehilim 49:11), "Yeva'aru v'Yichsalu" (Yirmeyah 10:8).

Malbim: Surely one who hates Musar he is a Ba'ar, i.e. one who is not prepared to receive Da'as. Also children fear punishment and accept Musar amidst fear. One who hates Tochachah, which is built on Sechel, one might have thought that he is not a Ba'ar; the verse teaches that he is a Ba'ar; he is considered like an animal. One who is proper to be called 'Adam', "Chai' and "Medaber', his nature is prepared to receive Tochachah, and in any case not to hate Tochachah, which comes to open the eyes of his Sechel.