After having sent his first letter to the church at Corinth, Paul was very anxious about the reaction that it could produce in the attitude of believers towards him. He was relieved when Titus brought him the news that the letter had been well received and obeyed.
Paul says he never did them wrong and wanted to have perfect communion with them, in life and in death. He was frank, because he loved them, and they gave him comfort and joy in the midst of the trials he was suffering.
Titus, to whom the letter with his name was sent some years later, was a faithful helper of Paul. He brought to Paul the news of how the Corinthians had favourably received the first letter (5-7).
Paul had repented for having written this letter, but now he did not regret it, because it had saddened them to the point of true repentance, which is to God (8-9).
Repentance toward God produces "salvation" - that is, spiritual restoration, which does not make anyone sad. But the "sorrow of the world" – that which does not come from sincere repentance - leads to death (10).
Here is another reason for the "comfort" Paul felt: Titus, who had brought the news of the sincere repentance of the Corinthians, was satisfied.
Thus, the great trust in the Corinthians that Paul had shown to Titus was justified, and which Titus himself had, after having visited them and been well received by them.
Thus, the confidence of Paul in the Corinthians was restored.
2 Open your hearts to us. We have wronged no one,  we have corrupted no one, and we have cheated no one. 
  
          3 I do not say this to condemn; for I have said before that you are in our  hearts, to die together and to live together.   
          4 Great is my boldness of speech toward you, great is my boasting on your  behalf. I am filled with comfort. I am exceedingly joyful in all our  tribulation. 
  
          5 For indeed, when we came to Macedonia, our bodies had no rest, but we were  troubled on every side. Outside were conflicts, inside were fears.   
          6 Nevertheless God, who comforts the downcast, comforted us by the coming of  Titus,   
          7 and not only by his coming, but also by the consolation with which he was  comforted in you, when he told us of your earnest desire, your mourning, your  zeal for me, so that I rejoiced even more.
          8 For even if I made you sorry with my letter, I do not regret it; though I did  regret it. For I perceive that the same epistle made you sorry, though only for  a while.   
          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, which 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 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. 
  
          13 Therefore we have been comforted in your comfort. And we rejoiced  exceedingly more for the joy of Titus, because his spirit has been refreshed by  you all.   
          14 For if in anything I have boasted to him about you, I am not ashamed. But as  we spoke all things to you in truth, even so our boasting to Titus was found  true.
             15 And his affections are greater for you as he remembers the obedience of you  all, how with fear and trembling you received him.
          16 Therefore I rejoice that I have confidence in you in everything.
2 Corinthians chapter 7, verses 2 to 16