The Sefas Emes asks a tremendous question. Chazal tell us that if the Beis haMikdash is not built in someone's lifetime it is as if he destroyed it. Our not having a Mikdash makes us as guilty as the dor of churban habayis. But how can that be? Surely it takes less zechuyos to maintain the status quo of having a Mikdash than it does to earn the Mikdash being rebuilt when you are starting from nothing!? We are starting in the hole, so to speak, with the deck stacked against us. Just because we haven't earned back the Mikdash doesn't mean we are as bad as they were!
Rashi interprets the list of places in the first pasuk of Devarim as tochacha -- these are all the things Klal Yisrael did wrong in the desert. One of the places mentioned is Paran, where the spies were sent from. One of the places is Chatzeiros, which Rashi (second pshat) explains refers to episode of Miriam's speaking lashon ha'ara, which the spies witnessed and did not learn from. Maharal in Gur Aryeh asks: doesn't this amount to the same tochacha two times? The word Paran is a hint that the spies spoke lashan ha'ra about Eretz Yisrael; the word Chatzeiros is a hint that the spies did not learn from Miriam and therefore spoke lashon ha'ra -- same thing?
Mahral answers that the failure to learn a lesson when given the opportunity to do so is itself a sin, apart from whatever wrongdoing that comes out from not having learned the lesson. The fact that the lesson that should have been learned from what happened to Miriam was not absorbed is itself a failure, irrespective of whatever happened later.
One of the big problems Yirmiyahu haNavi had is that no one took him seriously. Galus? Churban? Who is this guy kidding? I wonder if the generation that had the privilige of hearing the dvar Hashem from prophets and witnessing a functioning Mikdash even understood what this concept of galus. I wonder if the full horror of what Yirmiyahu was telling them could even register on their consciousness.
For better or worse, we now know what galus means and what Yirmiyahu was talking about. There are people alive still today, though fewer and fewer of them left, who saw firsthand what churban is all about. The dor that Yirmiyahu was speaking of could only relate to these ideas in the abstract, as some theoretical possible punishment -- we have seen the reality. For us not to learn from it, for us to not absorb the lessons of galus, would be a tragedy. We are not starting in the hole with the deck stacked against us. Aderaba, we are starting with the advantage of having been smacked with reality and taught a lesson that should inspire us to increased yearing for Eretz Yisrael, increaed tzipisa l'yeshu'a, and increased desire for geulah.
If that doesn't motivate us, if 9 Av doesn't inspire us, then indeed, we are as culpable as the generation of the churban. If it does motivate and inspire us, then this can hopefully be our last 9 Av in mourning.