When a Jew is about to break a glass at the climax of his wedding ceremony he recalls what King David said in his Tehillim about bringing up the memory of Jerusalem at the peak of his joy.

This Shabbat before Tisha BAv, Shabbat Chazon, introduces the saddest part of the year as we recall the destruction of the Beit Hamikdash and our long exile from our land. Breaking the glass expresses our sense of the incompleteness of any joy as long as our Sanctuary is in ruins. But it is also a reminder that even if a Jew thinks he has achieved all of his hearts desires he is still lacking the most important ingredient for his total success as long as he is not in Eretz Yisrael with a Beit Hamikdash to enable him to achieve total fulfillment.