Why was Paul so harsh with the Corinthians in 1 Corinthians?
Paul was harsh with the Corinthians in 1 Corinthians because their congregation had problems unlike any other under his apostleship. He had to deal firmly with those issues, including gross immorality, so the first letter accomplished its work and corrected them.
Les explains that this second letter is a follow-up to the first, where he had to deal with so many problems in the Corinthian church. They had problems like no other congregation under Paul's apostleship, and so he had to get rather hard with them in I Corinthians. Now in II Corinthians he comes back in defense of his apostleship, but also to encourage these people who had seemingly corrected their problems. The first letter had accomplished its work: they had dealt with the person involved in gross immorality, and he was evidently back in the fellowship. Because the message had come that correction occurred, Paul did not have to go personally as he had intended.