Answer:
   
   Even at the most joyous occasion in the couple's life, we are
   commanded by our sages to remember that our joy will never be complete
   until our temple is rebuilt in Jerusalem. Breaking a glass has become
   the traditional way to express this idea. Many grooms also place a
   small amount of ashes upon their head at the ceremony to express the
   same idea. Before breaking the glass, at some weddings a short sad
   song about the destruction of the temple is sung. The rabbis of the
   Talmud refer to this tradition as zecher l'chorban.
   
   Over time, this tradition has been updated to symbolize the
   Inquisitions, the pogroms, the Holocaust, and in our own time, the
   dangers faced by our brethren in Israel, and throughout the
   Middle-East. Some believe that the lesson is exclusively a Jewish
   lesson, but a human one: that no couple, no matter how much in love,
   has the right to separate themselves from humanity, but rather that
   each of us, as an individual, and as a family, is obligated, if only
   in some small way, to help bring the world a bit closer to what we
   call y'mei ha-mashiach, that messianic era, in which no peoples, and
   no persons, regardless of faith, regardless of origin, regardless of
   ethnicity, will ever again feel the heel of oppression.
   
   For some, there are other reasons. For example, the breaking of
   something lovely and fragile is sad. The measure of a good marriage is
   not only the ability to celebrate joy together, but also the ability
   to overcome the moments of sadness, of pain, of anxiety, of
   loneliness, of emptiness, of frustration, that enter every human
   relationship, by virtue our finite and therefore imperfect condition.
   Breaking the glass reminds us to celebrate the joys, as well as to
   overcome the moments of sadness.