This past Sunday, we began a three week mourning period over the destruction of the two Temples in Jerusalem. Jewish law dictates various practices whose purpose is to display our mourning. These restrictions become more intense in the last nine days of the three weeks and culminates with the fast of Tisha bav, the day of destruction of both Temples.
The reason the destruction of the Temples are so central in Judaism is because when the Temples were standing there was a great revelation of the Divine presence in the Temple. This revelation radiated from the Temple to the whole world. Thus, at the time of the Temple there was a great revelation of spirituality in the world. This will return on a yet higher plane with the coming of Moshiach who will rebuild the Third Temple.
Although, this three week period is a period of mourning is not considered a bad time. The destruction if the Temple was meant to arouse the Jews to return to G-d and abandon their sins. Once the Jews would return not only would the Temple be rebuild, but, we will be lifted to a much higher level then we were before the Temple was rebuilt. The reason for this is that whenever we have a downfall and then lift ourselves up the experience raises us to new heights. Thus, in the three week period, we focus on the purpose of the destruction and we increase in good deeds and Torah study with Joy which will lead us speedily to the third Temple with Moshiach.