Musar su Proverbi 27:5

The Torah next exhorts us not to bottle up our hatred against a fellow jew in our heart (19,17), but to tell him openly if we wish to criticize his conduct, i.e. הוכח תוכיח. Proverbs 27,5 provides the rationale, namely that a rebuke reveals loving concern for the person so rebuked. This is a virtue as explained by Rashi on Genesis 37,4 that Joseph's brothers, rather than concealing their dislike of Joseph and flattering him, spoke out about it. One must not contrast this with the statement of our sages who condemned Naval for saying what was in his heart. They even changed the reading of the name of his ancestor from נבל to כלבו so as to emphasize that every evil thing in his heart he expressed with his lips (Samuel 25, Jerusalem Talmud Sanhedrin 2,3). His was a special case; he should not have spoken at all instead of using his power of speech only to denigrate others. Joseph's brothers, on the other hand, had to maintain relations with him. Rather than pretend to like him they chose to say what was on their minds.