Regarding the sin of the meraglim, the Torah tells us that the entire nation cried that night. The Gemara (Taanis 29) explains that Hashem was going to bring us into Eretz Yisrael with amazing miracles; there was nothing to fear. But because we cried for no reason, Hashem gave us a reason to cry. 

R’ Biderman explains that if the Churban came about because we cried for no reason, we can derive that the Beis HaMikdash will be rebuilt when we recognize that there is no reason to cry.

When something disappointing happens, yet we believe that it is from Hashem, for the good, this awareness will rebuild the Beis HaMikdash!