Only the Temple of wood and stones was destroyed...

Even though seemingly the time of Bein Hametzarim [3 weeks between fast of 17 Tammuz and fast of Tisha B'Av] is a time of unhappiness since we recalll the destruction [of the Beis Hamikdash], but it is also obvious that this period of time is not a matter of sadness and depression, for what was destroyed was the [earthly] Beis Hamidash hatachton which was built of wood and stones, and not the spiritual Beis Hamidash within. And well-known is the saying of the sages on the verse "I shall dwell among them" -- amidst every single [member of the Jewish people].

The inyan of Bein Hametzarim is to recall and awaken us to the obligation of every single Jew to make for [Hashem] a dwelling place below through keeping the Torah and its Mitzvos in daily life, to fulfill what is written "and they shall make for me a dwelling place" and I shall dwell among them" mamash, to speed up the complete redemption via Moshiach Tzidkeinu by spreading the wellsprings, even among those who are presently "outside".

It is a great responsibility on each and every one of us in these days in particular, after the horrific destruction and ruin which we have seen before our eyes, and we whom Divine Providence has rescued and left us as a remnant in order give life to a numerous nation. And since the Holy One Blessed be He does not overburden any of his creatures, thus it is clear that together with this doubly great responsibility that falls on each and every one of us [men and women] is given also the powers and the abilities to each one to fulfill his or her task.

Letter of the Rebbe, 29 Tammuz, 5717 (1957) (Igros Kodesh v.15, p.300)