The gemara says that "If one mourns over Jerusalem he merits [zocheh] to see it's joyous rebuilding."
"Zocheh" is written in the present tense. Why? Mourning over Jerusalem doesn't bring one to merit rejoicing in the present - but in the future?!!
Ahhhh Rov Levi Yitzchak of Berditchev!!!: "Zocheh" ALSO means "zikuch" - purification. If one mourns over Jerusalem he will be so purified, that at that moment he will taste a little of the future joy of the building of Jerusalem.