Why use a pulley?

Scholars tend to say that the author uses the metaphor of
a pulley to talk about the forces that pull people up and down at the same time.  In a
pulley, a force that is pulling down, actually lifts something up at the same time.  In
this poem, the speaker is arguing that the same thing happens with
people.

He is saying that it is the forces that pull us
down to earth that also lift us to heaven.  The stresses of every day life are what pull
us down towards the earth.  At the same time, however, they are the things that make us
understand that we need God and therefore they pull us up towards heaven as
well.