How is "Pore'a Musar Mo'es Nafsho"?
Malbim: Musar scares a person with fear of punishment. The primary punishment is of the Nefesh in the world to come. One who rejects Musar, he is not concerned for his Nefesh and its demise. If so, he despises his Nefesh 1 ! Tochachah is via Sechel. Chasar Lev has no power that rules. He is like an animal that acts without heart, just according to the pull of the body and its desires. He will not acquire a heart via fear of Musar, for he does not know at all what is good and what is evil. [He will acquire a heart] only via afflictions - "v'Shevet l'Gev Chasar Lev" (10:13), like an animal, which is drawn via a staff.
Malbim (33): One who rejects this Musar, he despises his Divine Nefesh that is tied with Hashem, who oversees his deeds constantly.
How does one who hears rebuke acquire a heart?
R. Yonah: Kinyan ha'Lev is acquiring honored Midos and choices. One reaches a high level via hearing rebuke!
Malbim: If he hears rebuke, then he understands in his heart and acquires a heart that he lacked until now, like one who buys from a seller. He leans his ear to rebuke according to Sechel, which shows that Chachmah itself is good. There is honor for this - "v'Shomer Tochachas Yechubad" (13:18). He does not act due to fear, rather, due to his Sechel. This is via humility. A haughty person does not heed and hearken to rebuke.
