When one mourns the Beis HaMikdash, he draws an element of it to himself. The Beis HaMikdash was a place filled with joy, as we sing in the Shabbos zemiros, תוב למקדשך ולקדש קודשין אתר די ביה יחדון רוחין ונפשין, "Return to the Beis HaMikdash and to the Kodesh Kadoshim, a place where the souls rejoice" and with his mourning, he draws this immense joy to himself.
Some people are afraid to mourn because they don’t want to be sad, failing to realize that mourning results in the greatest joy.
Shulchan Aruch (554:25) states, כל המתאבל על ירושלים זוכה ורואה בשמחתה, "Whoever mourns over Yerushalayim merits and sees in its joy." זוכה ורואה is written in the present tense. The Kedushas Levi and other sefarim explain that when one mourns the Beis HaMikdash, he immediately experiences the joy of redemption. The Kedushas Levi (Eichah) writes, "When one thinks about holiness and mourns Yerushalayim, he immediately perceives an element of the joy of Yerushalayim, of how it will be in the future."
At a chasunah, we break a glass under the chuppah, we say the brachah שוש תשיש – a tefillah for the geulah, and the chassan wears ashes on his head, etc. These customs help us remember Yerushalayim and the Beis HaMikdash. The Sfas Emes (Ki Savo 5653) explains that the purpose of these customs isn’t to get us to mourn at a wedding; rather, they perfect the joy of the chasunah. We want the joy of the chasunah to be complete, but how can any happiness be complete in galus? So, we mourn, and the mourning draws the light and the joy of the era of Moshiach, which completes the joy of the chasunah.
The Sfas Emes writes, "At every simchah, one must remember the Beis HaMikdash… When the Beis HaMikdash stood, the simchah was complete. Today, we merit this simchah through mourning and yearning for the Beis HaMikdash. As it states (Yeshayah 66:10) שישו אתה משוש כל המתאבלים עליה, 'exult her in exultation, all you who mourned for her.' … Through his mourning, he will merit the joy of Yerushalayim."