Rebuke Needs Grace
To rebuke a fellow Jew successfully, one needs a light in the face and grace in the eyes of the listener.
At the heart of the Kohanim's blessing, "May Hashem make His face shine upon you and be gracious to you," the Lisa Rav hears a request for the one who guides others. The purpose of Klal Yisroel, he says, is to teach Hashem's ways and to rebuke each other when needed. The offerings in the Beis HaMikdash were lost, in part, because Jews failed to correct one another. But a correction only works when the listener can bear it, and for that the one who corrects needs the light of Hashem's face and a special charm in the eyes of those who hear him. Then people see that Hashem's name is called upon him, they respect him, and his words are accepted.
Source: Nachalas Yaakov, Bamidbar (Naso, Birkas Kohanim, "Yaer")
If there is something you need to say to someone, a child, a spouse, a friend or a student, do not start with the words. Begin by asking Hashem quietly to give your voice some grace and the listener an open heart. Then choose a calm moment and speak privately and kindly. Say what you hope for before you say what went wrong. Correction offered with warmth has a chance of being heard, whereas correction delivered in a hurry usually sets up a wall.