Rebbe Yitzchak of Neshchiz zy'a (Toldos Yitzchak) teaches, "Each year, during the days of the Three Weeks (Bein HaMetzarim), a very high level of chesed is revealed, for it is known that it was all for the good of Bnei Yisrael. Hashem poured out His wrath on wood and stone, and He didn't destroy Bnei Yisrael, chas ve'shalom. Immense love was aroused upon them, emanating from the uppermost, high worlds. Therefore, every year, once again, when these days arrive, this immense love is aroused and Hashem's love and compassion come to us and all of Bnei Yisrael."
The Midrash (Bamidbar Rabba 23:1) states, "Hakadosh Baruch Hu tells them, all the forty years that you were in the desert, I protected you, and you never had to run and escape. I had your enemies fall before you... There were many snakes and scorpions in the desert, as it states (Devarim 8:15) שָׂרָף ׁנָחָש וְעַקְרָב, 'snakes, vipers and scorpions', but I didn't allow them to harm you. Therefore, Hakadosh Baruch Hu said to Moshe, 'Write down their travels, where Yisrael traveled in the desert, so they will know about the miracles I performed for them in the desert."
The Ruzhiner zt'l said that his father, Reb Shalom Shachna of Prohibitsh zy'a, said that from the three weeks, he knows how the yomim noraim will be. And I say that the Three Weeks are like the diamonds that grow under the mountains of darkness. Although there is darkness, they shine brightly. When Moshiach comes, and the moon will shine as brightly as the sun shines, the Three Weeks will be yomim tovim. This is because the Three Weeks correspond to the רגלים שלש. Hashem yisbarach knows that when I was young and I had strength, each day of the Three Weeks, I felt a new light. Chazal (Brachos 60b) say, לטב רחמנא דעביד מה כל עביד, that everything Hashem does is for the good, and their intention was regarding the Three Weeks. These are days of yeridah for Yisrael, but it is all for the good."
Zu because he always said לטובה זו גם. The Darkei Teshuvah (Munkatch) zy'a quotes from mekubalim that the months of Tamuz and Av represent the eyes. In these months occurred the great churban, but it was undoubtedly for the good. Only we are like the blind who don't see and don't understand. Now, we only see the tremendous suffering, the bitter galus, etc. But in the future, we will see with our eyes the great chesed that was concealed within these tzaros. Nachum Ish Gam Zu was blind in both eyes, and he always said לטובה זו גם. This is a remez that even during the months of Tamuz and Av, which represent the eyes, and even if we are now blind and don't see Hashem’s kindness, but he should nevertheless proclaim with emunah sheleimah לטובה זו גם, this too is for the good.
