When there is only one supermarket in town, the owner isn't afraid to shout at his customers. He knows that his customers will return no matter how he treats them, as they have nowhere else to shop. But if a second supermarket opens, the storeowner will be very careful to speak kindly to his customers.
The lesson is about the importance of speaking kindly with children. We live in a world where there is competition. If a child feels unloved, he has other places to go, where he thinks he will feel loved (real or imaginary). Therefore, parents and educators must be cautious.
A rebuke is found in parashas Shemini (Vayikra 10:17), when Moshe rebuked Aharon for not eating the korban chatas. He said, הקדוש במקום החטאת את אכלתם לא מדוע, "Why didn't you eat the chatas....?" Take the first letters of these words. They spell אהבה מלא, full of love. This is because you first need to show someone you love him, and only then can you effectively deliver rebuke.
The elderly Reb Dov Yaffe zt'l (mashgiach of yeshiva Kfar Chasidim) was once hospitalized. The doctor wanted to prescribe a particular medication, but first, he asked Reb Dov Yaffe for a list of all his medications and whether he has an allergy to a certain medicine. Reb Dov Yaffe asked the doctor, "Why is it important to know all my medications? If you want to give me another medication, just give me a prescription."
The doctor explained that there are medications that clash with one another. One medicine might weaken the power of another medicine, and they might even be very dangerous to take both at the same time. When the doctor left the room, Reb Dov Yaffe said, "Did you hear how careful one must be before prescribing medication? We can say that the same is true when one wants to give mussar to rebuke someone. First one must know the person's situation well, precisely what he is going through, both in his ruchniyus and in the physical and social realm. All this information is necessary so the mussar he delivers will help and not cause any harm, chalilah."
One certainly won't accomplish anything if he rebukes his children or students with anger. It states (Vayikra 21:1) אמור משה אל 'ה ויאמר אליהם ואמרת אהרן בני הכהנים אל. Rashi writes that this pasuk is speaking to the adult kohanim, telling them that they must teach the young kohanim to be cautious with the halachos of kehunah. Reb Moshe Feinstein zt'l (Darash Moshe) asks why it doesn't state 'ה וידבר, which is the more common expression used in the Torah (see Makos 11.). The answer is the pasuk is discussing chinuch, and a great rule in chinuch is that one accomplishes more when he speaks with soft words. וידבר represents harsh words, but chinuch has to be with רכה אמירה, soft words.
