"One who mourns Jerusalem will merit to see it in its joy." [Ta'anit 30b]
At first glance, it seems strange that our Sages said that he will merit to see Jerusalem 'in its joy,' not 'in its rebuilding.' Is not the rebuilding primary and the joy secondary?
However, Rav Kook explained, our Sages knew that when the time would come to rebuild Jerusalem, everyone then alive — even those who did not mourn the destruction — would merit to see the reconstruction. The joy, on the other hand, would be felt only by those who mourned and grieved over the destruction, who yearned and ached for its renewal.
In the giddy days following the Balfour Declaration, he remarked:
"There are some Jews for whom the international recognition of the Jewish people's right to its land does not inspire joy. This is because the primary focus of their mourning is the spiritual destruction of Jerusalem and Eretz Yisrael, while the utter humiliation of the Land being subjected to foreign rule does not grieve them.
"But those who always felt deep sorrow, not only for the destruction of Jerusalem and Eretz Yisrael, but for the absence of Jewish sovereignty in our land, the international declaration that Land of Israel must return to the Jewish people is a source of happiness. They merit "to see Jerusalem in its joy.""
[from "Celebration of the Soul" by Rabbi Moshe Zvi Neriyah (translated by R. Pesach Jaffe), p. 266]