Why do we get so serious about mourning the destruction of the Bais Hamikdash? What is it about that jagged, overgrown Wall in Jerusalem that attracts our attention; that remains the focal point of world Judaism? And, most important of all: Why did Hashem - the Creator of everything, the One who lacks nothing and needs no space within which to "live" - choose to somehow limit Himself and dwell in our mikdash?
What does Hashem get out of our erecting a Bais Hamikdash? Nothing. He lacks nothing so there is no way we can add to His essence. What do we get from having a Bais Hamikdash? Now that's the question!
While there's more than one way to understand it, let's assume that we were created basically for our own good. In this world, we have the opportunity to excel in our single most important task: serving Hashem. As reward for a job well done, we are promised good things in the next world - the only world where we are capable of fully appreciating good things. It is, therefore, in our best interest to ensure that we become as perfect as humanly possible in order not to miss out on the real reward later.
Simply put, the Bais Hamikdash helps us do that. On one level, bringing an offering ( korbon) allows us to pick ourselves up after suffering a setback and redirect and rededicate our lives to the Divine service.
And in another sense, the very presence of the Bais Hamikdash and the knowledge of what went on there can serve to inspire us in our avodah. In many ways we were closer to Hashem while His house was functioning the way it was meant to.
Aren't these good reasons to mourn the destruction?