9 Now I rejoice, not that you were made sorry, but that your sorrow led to repentance. For you were made sorry in a godly manner, that you might suffer loss from us in nothing.
10 For godly sorrow produces repentance leading to salvation, not to be regretted; but the sorrow of the world produces death.
11 For observe this very thing, that you sorrowed in a godly manner: What diligence it produced in you, what clearing of yourselves, what indignation, what fear, what vehement desire, what zeal, what vindication! In all things you proved yourselves to be clear in this matter.
12 Therefore, although I wrote to you, I did not do it for the sake of him who had done the wrong, nor for the sake of him who suffered wrong, but that our care for you in the sight of God might appear to you.
I now am rejoicing, not because you were made to grieve but because you were made to grieve resulting in your repentance, for you were made to grieve in accordance with the will of God, in order that in not even one thing would you sustain injury or damage by reasons of us, for the grief which is according to the will of God achieves a repentance which leads to salvation, a repentance which has no regret. But the grief which is exercised by the world in its outworking results in death. For, look. This very same thing, this being made to grieve in accordance with God’s will, to what extent it produced earnestness in you, yes, verbal defense of yourselves, in fact, indignation, yes, fear, in fact, longing, yes, zeal, in fact, the meting out of disciplinary punishment. In everything, you showed yourselves to be immaculate in the aforementioned matter. [They had cleared themselves from the guilt of connivance with the case of incest by disciplining the guilty brother.] Therefore also I wrote to you, not on account of the one who committed the wrong [the incestuous son] nor even on account of the one who was wronged [his father], but that your earnestness which was on our behalf might be openly shown among you in the sight of God. — Wuest, page 427
Paul rejoiced at their earnest desire to carry out his commands; their grief because of the misconduct committed amongst them; their zeal against evil through affection for him (v.7); their carefulness in searching out the evil; their separation from it; their indignation against the evildoer; their fear of the anger of God; their desire for restoration; their zeal for holiness; and their exacting of punishment—all filled him with an overflowing of joy which was still further increased by union with the joy of Titus (v.13). — Williams, page 904.