Mueller poses this question during a downtime in chapter 5. Kropp immediately answers with, "Get out of this," and then get drunk. Then Kat reminds him of his wife and children back home.
Haie first talks about going home and finding a woman to jump in bed with, but then admits that if he were a non-com, he'd stay with the Prussians and serve out his time (essentially becoming a career military man).
Tjaden speaks of getting back at Himmelstoss, which the others agree would be easier if he became a lieutenant. Detering, the quiet farmer, remarks that he'd get straight back to the harvest.
But the boys who came to the war with Paul, the young ones without wives or children back at home, cannot imagine what to do with their lives after the war. They will not return to their parents' houses as "boys," but they haven't yet begun careers or families. Because of their war experience, they cannot imagine any of it.
We believe in such things no longer. We believe in the war. (Chapter 5)