In chapter 4, Paul talks about the change that takes place in the soldiers as they approach the front.
At the sound of the first droning of the shells we rush back...a thousand years. By the animal instinct that is awakened in us we are led and protected...One cannot explain it. (Chapter 4)
Here he explains that an animal instinct comes over them in the throws of battle that lead men to hit the ground just before a shell whizzes by. This unexplainable instinct is one attribute that Paul and others credit to their survival.
The other, is chance, or luck. In chapter 6 Paul talks about how he was playing cards in one dugout and left to visit friends in another dug out. Upon returning to the first, he realized that while he was gone, a direct hit had obliterated it. He decided to return to the dugout he had visited and arrived just in time to help dig it out. It had been hit while he was between the two. Paul admits that many soldiers feel chance plays an even bigger role in survival than instinct or skill.
Over us Chance hovers...It is this Chance that makes us indifferent. (Chapter 6)