From Waiting for Mashiach (#795)
Q: Why should we mourn for the churban Beis Hamikdosh (destruction of the Temple) if it was destroyed for our benefit?
A: Here’s a man whose appendix was removed from him. The appendix is not supposed to be removed; Hashem made the appendix for a purpose. It’s part of the lymph system, which system supplies antibodies to combat infection. But when the appendix becomes infected, you have to remove it, so he pays a lot of money to the hospital and to the professor who made the operation. He’s happy to be rid of his appendix, But he’s sorry that he had to remove it. We’re meant to have an appendix.
Therefore, we mourn for the churban Beis Hamikdosh. We’re sorry it had to be taken away from us. Why was it taken away? Because something went wrong. Either the royal house had to be abolished, or we had to get rid of the tzdukkim (Sadducees). If we hadn’t had them, we could still have the Beis Hamikdosh.
Hashem is preparing for us a much more glorious future
Therefore we mourn that it was done. If it hadn’t been needed, we’d be better off. Certainly we’re better off with a Beis Hamikdosh, but without tzdukim. Therefore, both things are true: we have to go through the formality of mourning, and nevertheless not to be broken hearted.
We mourn for the churban Beis Hamikdosh, but at the same time you're thinking how lucky I am! Tonight after Tisha B’Av, there's a big meal waiting for me: a big loaf of bread and a whole shaker full of salt and as much water as I want from the sink! I’m a happy man!
Think about that and thank Hashem. That’s what you have to think. If you won't enjoy what you have, then what are you asking "chadeish yameinu k’kedem — renew our days like earlier times."
We have to mourn for the churban Beis Hamikdosh, sit on the floor and think about what happened as much as we can, and yet you should know that Hashem is preparing for us a much more glorious future. When the time will come, it’ll be greater than ever before.