‘I now rejoice, not that you were made sorry, but that you were made sorry resulting in repentance; for you were made sorry after a godly sort, that you might suffer loss by us in nothing. For godly sorrow produces repentance unto salvation, which brings no regret. But the sorrow of the world produces death.'
For Paul's rejoicing is not in that he gave them pain, but in that it brought them to a change of mind and heart. They were made sorry in a godly way which produced ‘repentance' (a change of mind and heart, a turnaround) and which brought them not loss, but gain. They really in the end lost nothing by it, and they gained everything. Thus the source of his rejoicing.
For that is what godly sorrow does. It produces true repentance which results in salvation, and thus brings no regret. It is only the sorrow of the world, which has no good motive or result behind it, which has a deadening effect, and in the end produces only death. Godly sorrow is the spring of hope, and results in salvation and glory Worldly sorrow has no final hope, and is the harbinger of hopelessness and death. We note here again how quickly Paul can turn from present circumstances to a contemplation of the whole of God's saving work (compare 2 Corinthians 1:10), and the contrast between life and salvation, and death. (The contrast with death confirms that we are to see ‘salvation' as having its fullest soteriological meaning and not as just referring to wholeness).
He is not here saying that they had not previously been genuinely saved. He is describing the essence of genuine repentance which lies behind salvation, a repentance which must be reproduced continually in the face of (regretfully) continuing sin, so as to ensure the continual saving work that will finally present them perfect before God. Our first repentance is in one sense once for all (it changes the direction of our lives and results in our being within God's saving purposes) but there will then need to be continuing repentance in the face of continuing, although hopefully diminishing, sin, as we falter in the new way we have taken, and experience God's continual saving presence.
‘That you might suffer loss by us in nothing.' Some see this as more specifically having in mind loss of future reward, which is very possible. But it seems more probable that Paul means it in a general way which included any kind of loss, although clearly the idea of such future loss is a constant in Paul's letters (2Co 5:10; 1 Corinthians 3:12-15; 1 Corinthians 4:5; Romans 14:10-12) and is included. In the commercial world the verb zemiomai could refer to loss or damage in money or material goods due to unfavourable conditions or circumstances, such as the loss in goods and lives caused by a storm at sea. Thus the thought may include the havoc that discipline could have caused if over-applied. This was, as we saw earlier, Paul's concern for the offender whom the Corinthians continued to discipline even after he repented. Had the discipline continued, the man stood in danger of being overwhelmed by excessive sorrow (2 Corinthians 2:7). Paul had ensured that this had not happened to the Corinthians as a whole.
So to sum up why he rejoices,
1) The Corinthians' sorrow only lasted for a little while (2 Corinthians 7:8). They were not pained for any extended period of time, and so no permanent damage to the relationship occurred.
2) God's hand was evident in the church's response. They had become sorrowful as God intended (‘according to God' - 2 Corinthians 7:9).
3) The kind of sorrow that God intended and had brought about resulted in a turnaround, Your sorrow led you to repentance (2 Corinthians 7:9). They did not merely regret what they had done but repented of it, they were totally reoriented. This was demonstrated by the fact that they not only admitted that they had been to blame but also punished the offender (2 Corinthians 2:6; 2 Corinthians 7:11).
4) Most importantly the church was not harmed in any way by the severity of his letter (2 Corinthians 7:9).