It is written, “With righteousness shall you judge your fellow” [Acharei/ Kedoshim 19:15]
The Sages interpret this to mean, “Judge your fellow favorably” [Shevuot 30b]. How can we apparently lie to ourselves by judging people favorably in every case, when in certain cases we can see them doing the very opposite of something favorable? What is the meaning of this mitzvah in that case? The Sages have said, “Any man who is insolent will in the end stumble into sin” [Taanith 7b]. This means that shame serves as a barrier and an obstacle to sin. Once a person had breached the barriers of modesty and shame, there is nothing to prevent him from sinning, as it is written: “It is a good sign if a man is shamefaced. … No man who experiences shame will easily sin” [Nedarim 20a].
The same applies to a person’s influence on others. The first one who sins completely breaches the barriers of shame. The one who follows him does not require as much insolence to sin, and the third person needs even less, once these barriers have been broken down. This is why the sin of desecrating Hashem’s Name is so grave. A person who openly sins diminishes the intensity of the fear and shame that are engraved in man with regards to committing a sin, thereby prompting others to sin as well.
We can now understand how the advice given to us by the Sages, to judge others favorably, is designed to help us. It is meant to ensure that the barriers of shame are not breached within our own hearts, for once we are certain that everyone is righteous, how could we dare to be the first ones to sin? However if a person tries to find fault with everyone, he will be more likely to sin at a time of weakness.