how did paul think the men prevented themselves from going mad at the front line?

Basically burying his emotions and trying to keep a sense of cold detachment are the only things that can save a soldier's sanity at the front lines, "His emotions lie buried in the earth along with the soldiers who fell prey to them. His dullness protects him from going mad at the sight a slaughtered comrade or butchered friend." Witnessing terrible scenes can desensitise a soldier to the horrors of war. Paul welcomes this “dullness”, otherwise he will go crazy.