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I think that Paul does not want to give the soldiers any false hope that things might get better for them. Also once he is seen showing sympathy, the rest will certainly beg for the same. Paul can't handle that.
He hesitates about showing sympathy because thoughts and sympathy for the Russians could end up undermining him as a soldier. He can't think about sympathy and the fact that these men are just people sent to fight in the same way he is; he can't see them as anything but the enemy because anything more might render him unable to fight. It could even get him killed.
All Quiet on the Western Front
Here is a quote from GradeSaver that might help,
Similarly, Paul cannot muster true sympathy for the Russians because he sees in them only "the suffering of the creature, the awful melancholy of life and the pitilessness of men." He does not know the Russians beyond these generalities; as he says, "How little we understand each other." Such an understanding seems impossible under these conditions of scarcity, when each side wants to take advantage of the other for such bare necessities as boots and bread.