when Paul tells kemmerich's mother about her sons death, why is he surprised at her grief?

Paul is now so conditioned to the death he has witnessed that all he wants to do is make it easier. He doesn't seem to be able to understand the mother's tears or her hysteria. He grieves for the whole, not the individual...... it is no longer a matter of losing a close friend; it's become a matter of losing many.

From the text:

I cannot write that down. This quaking, sobbing woman who shakes me and cries out on me: "Why are you living then, when he is dead?"--who drowns me in tears and calls out: "What are you there for at all, child, when you --"--"

"--who drops into a chair and wails: "Did you see him? Did you see him then? How did he die?"

I tell her he was shot through the heart and died instantaneously. She looks at me, she doubts me: "You lie. I know better. I have felt how terribly he died. I have heard his voice at night, I have felt his anguish--tell the truth, I want to know it, I must know it."

"No," I say, "I was beside him. He died at once."

She pleads with me gently: "Tell me. You must tell me. I know you want to comfort me, but don't you see, you torment me far more than if you told me the truth? I cannot bear the uncertainty. Tell me how it was and even though it will be terrible, it will be far better than what I have to think if you don't."

I will never tell her, she can make mincemeat out of me first. I pity her, but she strikes me as rather stupid all the same. Why doesn't she stop worrying? Kemmerich will stay dead whether she knows about it or not. When a man has seen so many dead he cannot understand any longer why there should be so much anguish over a single individual."