After Bnei Yisrael conquered the nations living on the eastern side of the Yardein, the shevatim of Reuvein and Gad approached Moshe with a unique request. They stated that since they owned great quantities of livestock, and the newly conquered land was very fertile, they preferred to be allotted land on the eastern side of the Yardein rather than in Eretz Yisrael proper.
Moshe was upset with their request and the Torah (Bamidbar 32, 6-15) recounts his stern reply. He berated them for not wanting to enter Eretz Yisrael. He told them that their fathers were guilty of the same transgression and as a result that entire generation perished in the desert. "And behold you have risen up in place of your fathers, a group of transgressors, to add to Hashem's anger against Yisrael. For if you will turn away from Him, he will once again let them remain in the desert, and you will destroy this entire nation."
Rav Wolbe (Shiurei Chumash) notes that the Targum translates, "if you will turn away from Him" as "if you will turn away from fearing Him." Where exactly was their yiras shamayim lacking? All they wanted was that their portion of Eretz Yisrael should be allotted to them in the newly conquered area. If there was anything wrong with what they requested, it would appear to merely be a shortcoming with regard to bein adom l'chaveiro. How could they expect the entire nation to fight the many wars on the other side of the Yardein while they would sit complacently in the comfort of their homes? What does this have to do with a lack of yiras Shamayim?
Rav Wolbe answers as follows. When one sits in his house with all the shades closed, he sees nothing except for what is in the room together with him. If he would open the shades, he would be able to see everything going on in the street down below and in the sky up above. Similarly, when one's focus is directed inward he perceives nothing but him himself, and everyone else, whether other people or even Hashem, do not receive the proper attention. Only when a person shifts his focus outward does he become cognizant of both other people and his Creator. A flaw in bein adom l'chaveiro is an indication that there is also a flaw in one's yiras shamayim.
The Beis Hamikdosh was our connection to Hashem, and it was destroyed due to sinas chinom which is a flaw bein adom l'chaveiro. There is no better time than the Three Weeks to work on rectifying this shortcoming. Shifting our focus to others is the best way to help rebuild the Beis Hamikdosh and restore our connection with Hashem!