Jamieson, Fausset and Brown on 2 Corinthians 6:11

11. mouth ... open unto you--I use no concealment, such as some at Corinth have insinuated (2Co 4:2). I use all freedom and openness of speech to you as to beloved friends. Hence he introduces here, "O Corinthians" (compare Php 4:15). The enlargement of his heart towards them (2Co 7:3) produced his openness of mouth, that is, his unreserved expression of his inmost feelings. As an unloving man is narrow in heart, so the apostle's heart is enlarged by love, so as to take in his converts at Corinth, not only with their graces, but with their many shortcomings (compare 1Ki 4:29; Psa 119:32; Is 60:5).