The twelfth kinna read on Tisha B'Av is titled "O'hali Asher Ta'avta." This kinna was composed by Rav Elazar HaKalir, and each stanza concludes with a pasuk that ends with the word "po" (here). Rav Wolbe explains (Da'as Shlomo, Alei Shur vol. II pg. 409) that Rav Elazar HaKalir encapsulated the destruction of the Beis Hamikdosh in a single word, "here."
The Kuzari writes that the King of Kuzar asked an interesting question. Why is it that other religions (such as Islam) promise great reward in the next world, while the Torah hardly makes any mention about the reward that awaits us in the World to Come? The Chassid answered that other nations cannot guarantee anything with regard to this world. Therefore, all their promises revolve around the next world since there is no way for a human to prove such promises wrong. In contrast, the Torah does not need to rely on promises that will only be fulfilled in the world to come since it has the ability to guarantee the way our lives will look in this world. The greatest guarantee is Hashem's declaration, "And I will be for you a G-d and you will be for Me a nation" (Vayikra 26, 12). The Torah pledges that if we fulfill Hashem's commandments, we will have and feel a true connection to Hashem right here in this world.
This is what Bnei Yisrael lost with the destruction of the Beis Hamikdosh. Hashem's Shechina was here on earth for all to see, and when the Beis Hamikdosh was destroyed we lost this closeness and the resulting awesome spiritual levels. The profundity of this destruction grows as the galus lengthens, since it becomes more and more difficult to comprehend how Hashem could actually be found in this mundane mortal world. It used to be obvious that true fulfillment and pleasure in life could only be achieved through closeness to Hashem. We have since lost this clarity.
One of the early Rishonim, Rabbeinu Tam, writes that every person experiences "days of love, and days of hatred and despair." Some days our avodas Hashem seems to flow easily, while on other days we feel sluggish and every aspect of avodas Hashem seems like a heavy load. Rav Wolbe says that this is because on days that awareness of Hashem's closeness is felt more strongly it is easier to fulfill His dictates, while the opposite is true on days when we are less cognizant of His closeness.
The Beis Hamikdosh was a point of connection. It was the site where the physical connected with the spiritual, and it was abundantly clear to all how they were intertwined. The destruction caused the spiritual to become separated from the physical, and from then on it became increasingly difficult to discern spirituality here in this world. Nevertheless, despite the many years of exile and destruction, Klal Yisrael is still aware that Hashem does exist here. We merely must make an effort to look for Him.
Tisha B'Av is the day which was given to us as an opportunity to contemplate just how far we have drifted from Hashem's Shechina which (even today) can be found here on earth. It is amazing to think that Tisha B'Av is actually called a mo'eid. This is because we are able to realize that despite all the destruction, Hashem can still be found right here on Earth! May we merit the ultimate closeness to Hashem with the rebuilding of the Beis Hamikdosh speedily in our days!