Why is 2 Corinthians so different in tone from 1 Corinthians?
The tone shifted because 1 Corinthians confronted a church full of problems that Paul had to address, while 2 Corinthians, written within a year, encouraged and exhorted the same believers instead of condemning them. He even boasted about them to other churches and instructed them on grace-age giving.
Les explains that in his first letter to the Corinthians, Paul had to deal with a church full of problems and address them, almost feeling bad about it. Within about a year he wrote the second letter with a whole different attitude: he was no longer condemning them but encouraging and exhorting them, even boasting about them to the other churches in Achaia and Macedonia. He then addressed a still-touchy subject—their billfold—with chapters 8 and 9 giving instructions on Christian grace-age giving.