Westhus and Detering are old war dogs by the time Paul and his friends reach the 2nd Company. When we first meet them, they have little concern about Paul and his friends, likely believing the noobs won't last long. But as time goes on, both men learn to accept the young replacements as members of 2nd Company.
As older men, they provide a contrast for the younger soldiers of the 2nd Company. Whereas the youngsters have put their future plans and dreams on hold, both Westhus and Detering had established lives, jobs, and families before taking up arms to fight for the fatherland. While all of the 2nd Company suffers for the war effort, Detering's and Westhus's suffering is more a desire to return to the everyday, rather than acclimating to the life of a soldier.
For example, when the soldiers wonder what they'll do during peacetime, the young dudes have young dude considerations: they want to get drunk, and wonder whether it's worth returning to finish school. But Westhus wants to return to his job in the peat fields.
Detering is especially prone to homesickness and nostalgia, as evident in the following exchange:
WESTHUS: We passed a cherry tree and when he saw it, he sort of went crazy. I could hardly drag him away.
DETERING: It was beautiful. I have a big orchard with cherry trees at home. And when they're in full blossom, from the hayloft it looks like one single sheet. So white.
ALBERT: Perhaps you can get leave soon.
LEER: You may even be sent back as a farmer.
DETERING: A woman can't run a farm alone. That's no good, you know? No matter how hard she works. Harvest coming on again.
Detering's homesickness manifests as a type of mental breakdown, worrying about his wife, farm, and the life he left behind. This makes him unique—the other soldiers don't have to worry about the warfront and the homefront simultaneously.
Yet despite their differences, both Westhus and Detering share similar fates to their young comrades. When Paul returns to the Front after his furlough, Tjaden updates him on the casualties of the 2nd Company. Westhus went out to save a wounded messenger dog (aww!) and was killed.
And Detering? Well, let's just let Tjaden fill you in:
"I guess he never got over [the cherry blossoms]. He started out one night to go home and help his wife with the farm. They got him behind the lines and we never heard of him since. He was just homesick but probably they couldn't see it that way."
The implication is that the top brass saw it as desertion, an offense potentially punishable by death.