An old joke goes that when G-d gave the Torah to the Jews, everybody started dancing and rejoicing. The Chasidim, who woke up late, did not see what had preceded this and so thought that the dancing was the Torah. Upon deeper analysis, however, the Chasidim in the joke might have been right – in an ultimate way, the laws of the Torah remain a means and not an end in themselves. The end is an elevated universe wherein man is in harmony with his Maker and the means is the observance of His laws.
Since the laws and customs of the three week bein hametzarim period are modeled on the laws of aveilut (mourning), it is difficult to conjure up the appropriate thoughts unless one has been through personal aveilut. My own mourning for my father a few years ago allowed me to better understand what is to be felt as we come closer to Tisha B’Av.
Mourning is a sadness of an almost irreparable nature. When a person dies, certain opportunities are lost to those around him. That sense of loss brings regret at not having taken advantage of what is no longer available. Outside of death, this is a highly uncommon situation, as most opportunities are retainable or re-creatable.
It is true that Judaism is generally about repair and not regret. We work on improving ourselves and our world. To let thoughts fester on what went wrong in the past is missing the point – all is geared to the practical goal of getting it right the next time around.
Nonetheless, all human emotions are designed to serve us in our spiritual quest. There is even a need to expose ourselves to the spiritually dangerous realm of finality. (Regarding the danger caused by awareness of finality, see R.S.R. Hirsch, Veyikra 11:46-47 on the relationship between death and ritual impurity. This is because our natural refusal to internalize finality brings about an irresponsible optimism that allows us to miss the seriousness of any given opportunity.
The Jewish people have irreparably lost great spiritual opportunities. The second Temple was not as spiritually elevated as the first. Various midrashim that speak about the messianic period associated with the third Temple give us the impression that it will be established as a result of less human merit than in the past. (It is interesting to note that the second Temple became more physically spectacular under Herod, even as it became more devoid of spirituality. Thus, I am not sure what to make of descriptions of the third Temple being the most physically glorious of them all.) So too, in each generation, the potential messiah is likely to be of lesser stature than his predecessor.
Focusing on the past allows us to realize the potential for loss in the future. Mourning is about that focus which is the internalization of loss. In the penultimate page of Ta'anit, it is stated that anyone who mourns for Jerusalem will merit and see its joy. The halachot are a vehicle – the point is the mourning. One who has emotionally internalized the irreparable losses of the past is able to appreciate the gift of new opportunities. One who has not may sleep right through their joy.