What are we mourning during these 3 weeks? An ancient shul?
We live in a world where little children are sold [by their own family members!] into slavery and prostitution. Where there are people who are starving, where disease abounds, where indescribable evil is perpetrated by animals masquerading as human beings. There is so much suffering.
Hashem originally put Adam and Chava into gan eden to teach us that a place of UNLIMITED PLEASURE is where man belongs. Yet today we are so far removed from there.
The beis hamikdash means that Hashem's sweet, warm, loving presence is felt on earth, resulting in a blissful existence for all.
THAT is what we are lacking. Some people live a privileged existence so they don't feel the destruction so much, but opening up ones eyes will make one realize how much we need Him back here with us.
Our job as Jews is to bring Him back.