The verse says Rejoice with trembling (Psalm 2:11). The rabbis took that seriously. If joy goes unchecked, they feared, it becomes carelessness, and carelessness forgets that the Temple is still in ruins. So they invented small theatrical shocks to keep the trembling in the rejoicing.
Mar, son of Ravina, threw a grand wedding feast for his son. When the rabbis were laughing too hard, when the music had climbed too high, he walked in holding a crystal cup worth four hundred zuz, a small fortune. He smashed it on the stone floor in front of them. The laughter stopped. Sorrow and trembling filled the room. And then the joy returned, but with a memory of loss underneath it.
Rav Ashi did the same at his son's wedding. Seeing the rabbis too far gone in jubilation, he brought out a precious white glass cup and broke it.
On another wedding night the rabbis turned to Rav Hamnuna and called, Give us a song! He stood and sang: Woe to us, for we must die! Woe to us, for we must die! Startled, they asked, What should the chorus be? He answered: Alas, where is the Torah we have studied? Where are the good works we have done? Let them protect us from the judgment of Gehinnom.
Rabbi Yochanan, quoting Rabbi Shimon bar Yochai, pushed this even further. It is forbidden, he taught, for a person to fill his mouth with laughter in this world, for it is written (Psalm 126:2), Then, meaning in the world to come and not before, shall our mouth be filled with laughter. Resh Lakish, from the day he accepted that teaching, never once let a full laugh escape him again.
This cluster of scenes from tractate Berakhot 30b-31a, preserved in Hebraic Literature (1901), is the origin of the smashed glass at the Jewish wedding. Joy without memory is not Jewish joy. The crack under the foot keeps the dance honest.