R. Yonah: This is (a) lest a stain of sin remain that reduces the love ¹ , (b) to increase the humility of his heart, (c) lest serenity decrease his fear, and (d) in order to increase his reward. No good in this world can equal this!

Malbim: Tochachah is a sign of love. Amidst love, He oversees him to improve his way and raise him to higher levels. He withholds from him excesses and delicacies and leans his shoulder to bear the yoke of Torah, constantly learning

Rashi: A father wants to benefit his son, and he appeases him after he strikes him ¹ with a staff. So the good will be pleasant to you after the affliction.

R. Yonah: It is like a father rebukes a son whom he loves. If a father has many sons, the one whom he loves the most, he rebukes him more than all his sons and he is diligent to give to him Musar, so all his Midos will be good and there will be no [spiritual] blemish in him. Therefore, Hashem rebukes one who He loves more than commoners - "Hashem Tzadik Yivchan" (Tehilim 11:5).