R. David Derovan in an e-mail devar Torah:
Why do we mourn the destruction of the Beit Hamikdash? We are certainly not mourning the loss of a building, per se. Rather, the Beit HaMikdash was the place where the Jew encountered God. Just to walk through the gates and to witness the various ceremonies and rituals was to walk, literally, into God's house and to be in His presence. Theoretically, every active Mitzvah offers us the same experience, but we all know how terribly difficult it is to feel God's presence while performing a Mitzvah. Not so, the Mikdash. The experience in God's house was one of heightened consciousness to the point that we encountered him just by being there. And that is what is missing in our lives. It is the loss of that experience and encounter with God that we mourn. So we pray that our eyes will literally see God's return to Zion!