Why did Mary Shelley include Percy Shelly's poem in volume 2, chapter2?

Mary Shelley included the poem
Mutability by Percy Bysshe Shelley (her husband) because the
central theme of the poem is intrinsically connected with the fate of Victor
Frankenstein. 

We rest; a dream has power to poison
sleep.
   We rise; one wand'ring thought pollutes the day.
We feel,
conceive, or reason; laugh, or weep,
   Embrace fond woe, or cast our cares
away;
It is the same: for, be it joy or sorrow,
   The path of its
departure still is free.
Man's yesterday may ne'er be like his
morrow;
   Nought may endure but
mutability!

Mutability is the essence of life: It is the
combination of all the elements and circumstances which directly affect our destinies
and change our lives forever.  At first glance the poem does not seem to fit in the
narrative at that specific point in the story. Yet, if we analyze the message of the
poem, we realize that mutability has indeed been ever-present in the
story.

From the very beginning we see that something big
will happen to Victor and that it may not necessarily be something good. His hunger for
learning, his obsession with the idea of creating life, and his intensity of character
can help us predict that a huge change is in the horizon for him. Hence, mutability is
the source of that change which not only affected Victor, but also Elizabeth, Henry,
William, and Justine.