Now I rejoice, not that you were made sorry, but that you sorrowed to repentance: for you were made sorry after a godly manner, that you might suffer loss by us in nothing.
Read Chapter 7
Ambrose of Milan
AD 397
Hence Paul teaches us that that kind of sorrow is of value which has not this world but God as its end. It is right, he says, that you become sorrowful, so as to feel repentance in the face of God… . Take note of those who in the Old Testament were sorrowful in the midst of their bodily labors and who attained grace, while those who found delight in such pleasures continued to be punished. Hence the Hebrews, who groaned in the works of Egypt, attained the grace of the just and those “who ate bread with mourning and fear” were supplied with spiritual good.
Ye sorrowed to repentance. This sorrow led you to repentance, to mourning (ver7), to indignation and revenge (ver11). Repentance, therefore, is not merely a coming to one"s self again, as I will show directly by several proofs.
Now I rejoice I should have been inconsolable, had my letter made you sad, without producing the salutary effect intended by it; but I now rejoice that it caused a sorrow and sadness productive of the great advantages you have reaped from it. Thus in every sentence St. Paul shows the solicitude of a father, seeking nothing but the advancement of his spiritual children. (Calmet)
Like a father who watches his son being operated on, Paul rejoices not for the pain being inflicted but for the cure which is the ultimate result. He had no desire to cause harm for its own sake.