. Why does Paul say that “the front is a cage”? Why does Paul believe in Chance?

The quote goes like this:

"The front is a cage in which we must await fearfully whatever may happen. We lie under the network of arching shells and live in a suspense of uncertainty. Over us, Chance hovers. If a shot comes, we can duck, that is all; we neither know nor can determine where it will fall...."

The soldiers feel trapped at the front. The battle is like a storm and they are imprisoned in the vortex. There is no such thing as dodging or ducking a bullet. Whether they live or die is simply left to chance.