The Torah commands us: Judge your fellow man justly (Vayikra 19:15). This is primarily an obligation for a judge to administer fair justice, but it also includes an obligation for every Jew to judge his fellow man favorably. This is called judging “justly” because every righteous Jew deserves to be considered innocent until proven guilty (see Torah Temimah). The halachic obligation to judge others favorably depends on their level of righteousness and the circumstances (see Chafetz Chaim,Pesichah, Be’er Mayim Chaim §3).

Sefer HaChinuch writes that the two parts of this mitzvah, judging justly in a court and judging people favorably, are complimentary, for the former upholds the justice of society and the latter upholds the peace of society. When people are willing to give others the benefit of the doubt, they do not feel resentment toward others, for they assume that anyone who has slighted them must have done so unintentionally, or that he must have subsequently regretted his actions (Ahavas Yisrael §5).

Semak (§8) includes the mitzvah to judge others favorably within the mitzvah to love one's fellow Jew like himself. The Chafetz Chaim explains (Shemiras HaLashon, Shaar HaTevunah §5) that just as a person would want others to see his actions in good light, he should do the same for others. Chassid Yaavetz (1:6) calls this the greatest of kindnesses.

Maharil Diskin (Vol. I pg. 54b) notes that judging others favorably is also beneficial for oneself, because a person tends to act according to the standards of his society. Thus, if he thinks that everyone around him is acting incorrectly, he will not restrain himself from doing the same, but if he judges others favorably and considers them righteous, he will be ashamed to lower himself in their eyes (cited in Maadanei Yom Tov, pg. 62).

When we judge others favorably, Hashem judges us favorably (Shabbos 127b). Although Hashem has no doubts as to the circumstances of a person's sins, there is nevertheless room for Him to judge him leniently and focus on his good intentions, as long as that person regularly does the same when he judges others (see Sifsei Chaim, Mo’adim Vol. I pg. 219). It emerges that when we judge others favorably, we ultimately benefit ourselves.

In Parashas Shemos, Moshe told Hashem that the Jewish people would not believe that Hashem was about to redeem them, and He struck him with tzaraas (leprosy) as a punishment for suspecting them falsely. The Sages derive from this that whoever suspects his fellow Jew falsely is stricken on his body (Shemos Rabbah 3:13). His own body deserves to be blemished as a punishment for blemishing his fellow Jew’s character (Yefeh Toar). From the fact that Hashem exacted this punishment upon a man as great and saintly as Moshe, we should take to heart how important it is to Him that we see the good in our fellow Jews.