Our Sages said: “Do not be scornful towards any person, for there is no person who does not have his hour” (Avos 4:3). That is, one should not disgrace or curse even the lowliest heathen, and all the more so a Jew — for God made man with His own image (Bereishis 9:6). One who disgraces a Jew is [thus] considered as if he disgraces the Shechinah (the Divine Presence), Heaven forbid. The only person one may look at with scorn is one’s self, since a person knows his own blemishes and imperfections. But the way of the world is the exact opposite: a person sees only the faults of others and not his own. This is because it is human nature to look for other people’s faults and then immediately mentally condemn them. Of course, when it comes to one’s self, the person will go to great lengths to rationalize and cover-up his shortcomings and convince himself that his deeds are pure and there really is no one like him in the whole Earth!
But this is not the correct path. Rather, it is proper for an upright person to judge everyone else favorably, but himself as guilty; to examine one’s deeds to make sure that there isn’t the slightest impure motive. Even if a person learned much Torah and reached astonishing achievements in the performance of mitzvos and good deeds, it should all be insignificant in his eyes when he compares them to what was expected of him, given his level of insight and abilities. [He must also keep in mind] his inadequate personal value and carefulness in doing mitzvos with the proper intent, fear, love, and great joy. Even if he possesses the merit of mitzvos and good deeds, he must always be concerned that perhaps what he “owes” [Heaven] is greater than his rewards [from his good deeds].