Titus , who had observed the Corinthians" repentance, had increased Paul"s joy further by reporting that to him. Paul"s words of praise for his readers before he had sent Titus to them had proved worthy in view of their response to Paul"s messenger and his message. [source][source][source]
"Paul"s relief stemmed from the fact that his generous assurances to Titus about the Corinthians had not proved unfounded and therefore embarrassing ( 2 Corinthians 7:14). On the contrary (alla), just as his own truthfulness had been vindicated at Corinth (cf. 2 Corinthians 1:18-20), so also his boasting about them had now proved fully justified." [1][source]
"Paul"s attitude shows how excellent a director of their spiritual interests he is: not hesitating to reprove what is amiss, but yet warmly and sympathetically encouraging them in the true emotions of those whose hearts are regenerate, which is the best way of ensuring that their past errors will not be repeated." [2][source]