A. There is an old witticism, “When the bridegroom breaks the glass it is the last time he can put his foot down”. The folklorists connect the custom with the consummation of the marriage. Homileticists say, “As one step shatters the glass, so will one act of unfaithfulness for ever destroy the happiness of the home” (JH Hertz).
The generally accepted explanation is that it recalls the destruction of the Temple and ensures that even in the midst of joy we remember the tragedies. The Talmud reports that when Mar, son of Ravina, made a wedding feast for his son the guests were in an uproarious mood, so he took a costly vase and shattered it in order to curb their spirits (Ber. 30b). Tosafot says this is the origin of breaking the glass at weddings.