The Nefesh Hachaim (Sha'ar Aleph Chap. 4) has a novel approach to understanding the Beis Hamikdosh and its destruction. He writes that Hashem commanded, "Make for a Me a mikdosh so that I can dwell within them." Chazal note that Hashem did not say, "So that I can dwell within 'it'." Rather, He said, "so that I can dwell within them," since He desires to dwell within each and every person from the Jewish Nation. Every person can become a veritable beis hamikdosh, as we find that the Navi (Yirmiya 7) refers to the righteous as "the sanctuary of Hashem." The commandment to build a Beis Hamikdosh out of stone was to facilitate an understanding of how our personal beis hamikdosh should look. Actions carried out with pure and holy intentions and with the goal of creating an abode for Hashem in this very physical world, mirror the holy vessels of the Beis Hamikdosh, which were prepared with the express objective of creating a place for the Shechinah to reside on Earth. Hence, when the actions of Bnei Yisroel deteriorated to the point that they desecrated their internal beis hamikdosh, the external Beis Hamikdosh automatically lost its viability and was destroyed.
Hence, Rav Wolbe says that the mourning of Tisha B'av and the days leading up to it is not merely mourning the Beis Hamikdosh that once stood in Yerushalayim. We are mourning the destruction of our internal greatness. Is there anyone who truly believes that he has the ability to turn himself into an abode for the Shechina? Bnei Yisroel has always produced great leaders from whom we could get a glimpse of true greatness. One such example is the Chofetz Chaim. Every action of his was measured and every word was weighed. He was a Torah giant, a paradigm of kindness and humility, and his tefillos were one of a kind. Every Jew has the ability to reach a similar level. One of the obstacles in our generation is the mindset that people are small and will always remain small. We must rid ourselves of this mindset, because it destroys our personal beis hamikdosh.
We do not recite tachnun on Tisha B'av because the pasuk refers to it as a mo'ed. We can understand why Sukkos is a mo'ed; but why is Tisha B'av a mo'ed? Rav Yeruchom Levovitz explains that there are moadim of kiruv - closeness, and moadim of richuk - distance. Rav Wolbe elaborates that Tisha B'av is a time for one to contemplate just how far he is from Hashem. How in reality, his every action has the ability to create an atmosphere suitable to house the Shechina, and yet, he doesn't believe for a second that he can achieve such a goal. Recognizing and acknowledging one's distance from Hashem, is the first step in rebuilding our internal beis hamikdosh. Once this is accomplished, the Beis Hamikdosh in Yerushalyim will follow immediately. That is why Chazal said, "All who mourn Yerushalayim will merit seeing its joy."