Why does Kropp feel that "The war has ruined us for everything?"

Albert means that the war took away their youth and innocence. The following quote says it best:

"He is right. We are not youth any longer. We don't want to take the world by storm. We are fleeing. We fly from ourselves. From our life. We were eighteen and had begun to love life and the world; and we had to shoot it to pieces. The first bomb, the first explosion, burst in our hearts. We are cut off from activity, from striving, from progress. We believe in such things no longer, we believe in the war."