This change happens throughout the book. I'll try to give you a few more quotes.
"We march up, moody or good-tempered soldiers – we reach the zone where the front begins and become on the instant human animals." (Ch 2 pg.27)
"I imagined leave would be different from this. Indeed, it was different a year ago. It is I of course that have changed in the interval. There lies a gulf between that time and to-day. At that time I still knew nothing about the war, we had only been in quiet sectors. But now I see that I have been crushed without knowing it. I find I do not belong here any more, it is a foreign world." Chapter 7 pg 173