How do the themes/story of the old woman contribute to the overall message in Voltaire's Candide?

In Voltaire's novel, Candide, the
three themes that stand out in the old woman's tale for me
are:

• one cannot depend that things will always
  stay the same: being rich and famous one day does not necessarily guarantee that this
  will not change


• even though we may believe our troubles
  are awful, there is always someone with difficulties worse than our
  own


• even when we hate our lives and think it would be
  better to be dead, we still hold on to life; while there is life, there is still
  hope

Voltaire
wrote Candide in the form of a  href="http://web.cn.edu/kwheeler/lit_terms_P.html">parody (a form of
satire) directed toward Gottfried Wilhelm von Leibnitz and his philosophy on optimism—we
see this in Pangloss' teachings and attitude which continually return to his belief that
in every situation, "this best of all possible worlds." In other words, everything is as
it should be.

My sense is that more than having an
unshakable belief that things are always for the best no matter the circumstances, I
imagine that Voltaire believed a realistic approach to life was more valuable. This is
not to say that people should not have an appreciation for positive things in their
life, but one need not appreciate difficulties and hardships,
seeing them as positive parts of our human
existence.

Because Voltaire is making fun of optimism under
every circumstance, I believe the old woman's story, and the themes found there,
encourage the reader to feel positive when appropriate, and sad when a situation
dictates it. Bad things happen, but this is not something we must accept without
resistance or feeling.

The ideas that things can change at
any time, there are always other people with problems worse than our own, and regardless
of our heartaches, life is still valuable are realistic concepts that follow the advice
that the old woman shares: she is not silly enough to believe that watching her mother
murdered was "the best of all possible worlds." And this is exactly what Voltaire was
trying to share with his audience. The author uses the character of the old woman to
allow us to feel honestly about the things that fill our life: thankful when things are
good, and hopeful when things go awry.