When Kemmerich's mother learns of her son's death, why is Paul surprised at her grief?

Paul is surprised at the intensity of Kemmerich's mother's grief because his own senses have been completely dulled by his experiences in the war.  He has seen killing, in all its horrifying varieties, everyday; in order to survive, Paul's own feelings have been inured to death.  When Kemmerich's mother responds to the news of her son's demise with "quaking (and) sobbing" hysteria, Paul is a little taken aback.  Although he had known that the knowledge would be hard for her to bear and had retained enough sensitivity to hold back the grisly details of Kemmerich's last days, his pity for her is mixed with the feeling that she is "rather stupid all the same".  Jadedly, Paul wonders why she is taking it so hard, because, practically speaking,

"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".

In his frustration at her reaction, Paul rather tersely tells Kemmerich's mother that her son's death was intantaneous and without suffering.  She does not fully believe him, however, and makes him swear "by everything that is sacred to (him) that he speaks the truth.  Paul acknowledges to himself that after all he has been through, there is nothing in the world that is sacred to him anymore, and has no problem swearing to Kemmerich's mother that her son "died at once", even though it is not true (Chapter 7).