The Previous Rebbe said:
Even if the other person is not as he should be, the person should know that he is lacking. The other person doesn’t know what you know. If he would know what you know, he would certainly become a different person. If you would behave properly, others would learn from you.
Sefer HaMa’amarim 5710, p. 264.
In my own words: When one see a fault in others, human nature is to see it as just that—a fault in him, and not in oneself. But in many cases in fact the opposite is true: The other guy behaves inappropriately because he doesn’t know better. But I do know better, and yet I fail to live up to what is expected of me. Moreover, if I would change, he would be inspired. So in a way, my negligence is the cause of his faults.