After the destruction of the Beit HaMikdash, the Jewish exiles were taken to Bavel. Sitting alongside the rivers of Bavel, the Jewish prisoners were taunted by their captors to, "Sing for us your songs of Zion." This tragic event was eternalized in the chapter of Tehillim, "Al naharot bavel". The Jewish response to this mockery were the words that have been repeated for thousands of years, "Im eshcachech yerushalayim tishcach yemini."
Chazal explain that the Babylonians were not merely asking the Jews to play music, but they were making fun of their inability to continue to sing in the Beit HaMikdash. When we recite, "Al naharot bavel," we are commemorating the tragedy that the, "Shir shehayu haleviim omrim beveit hamikdash", no longer exists. What is so unique about this loss that causes it to warrant its own form of mourning?
In Devarim 28:47 we are told that we have an absolute obligation to perform avodat Hashem with joy. Chazal in Erchin 11a see two levels of interpretation when explaining what constitutes this mitzvah. In addition to the overall obligation to observe all mitzvot in a joyous state, this joy is primarily expressed in the Beit HaMikdash, specifically through the songs that accompanied the korbanot. This shira set the tone for each individual's avodat Hashem. Once the Beit HaMikdash was destroyed, the communal expression of avodat Hashem besimcha could no longer exist in the fullest sense, detracting from each individual's ability to reach the ultimate level of simcha.
Prior to the churban, there was already a lack of excitement in the daily avodat Hashem of each individual. One of the factors to which Chazal attribute the Churban (Nedarim 661a) is not that Jews were not learning sufficiently, but that they failed to recite a brachah prior to their learning. Reciting a brachah indicates that one appreciates the gift he is receiving. One who learns but fails to say a brachah clearly does not approach the mitzvah with the proper enthusiasm. An act of avodat Hashem that lacks a brachah is not an avodah besimcha. The lack of simcha in the Jews' avodat Hashem helped bring about the churban. After the churban it became even more difficult to attain the correct state of mind because the source of simcha, the shira of the Beit Hamikdash, was gone.
Although the situation is bleak, it is not hopeless because we can still achieve simcha in our avodat Hashem. We have to rectify the mistake that caused the churban by reciting birkat ha-torah in the fullest sense. We must involve ourselves in limud ha-torah with excitement and joy. Just as a lack of enthusiasm brought about the destruction of the Beit Hamikdash, may we merit to see its rebuilding brought about through our rededication to avodat Hashem besimcha.