:ט Let the Building bear witness

שאלו את רבי אלעזר: ראשונים גדולים, או אחרונים גדולים? אמר להם: תנו עיניכם בבירה. איכא דאמרי, אמר להם: עידיכם בירה

I found this exchange most intriguing. The גמרא is comparing the merits of the generations that saw the destruction of each בית המקדש. The consensus seems to be that the fact the בית המקדש has not been rebuilt, in contrast to the relatively short initial exile of 70 years, is proof of the greater merit of the earlier generations.

We are taught (actually ירושלמי in this פרק)

כל דור שאינו נבנה בימיו מעלין עליו כאילו הוא החריבו

It would seem that the responsibility for bringing the בית המקדש back would lie in the hands of the subsequent generations in exile. Yet, the גמרא seems to tie it back to the generation in which it was destroyed. I suppose one support for this could be that the prophecy of the 70-year exile was already given to ירמיהו כ"ט:י. The גמרא must understand that it was due to the merit of the generation of the destruction that an expiry was put on the ensuing exile from the very beginning, whereas as no such favour was granted the second time.