Paul has a difficult time remembering the past because of the contrast between the calm in the memories and the chaos of war.
On the front, there is always noise. In their memories only, there is “stillness.” Paul says that memories awaken “a vast, inapprehensible melancholy” in him.
But here in the trenches they are completely lost to us. They arise no more; we are dead and they stand remote on the horizon, they are a mysterious reflection, an apparition, that haunts us, that we fear and love without hope. (ch 6, p. 121)
Paul feels that the memory is strong, but “unattainable.” He describes the soldiers as dead. They can’t even remember things peacefully, because when they remember all it does is remind them how terrible their current situation is. Therefore even these happy memories of the past terrorize them.
Memories are just another thing that the war has taken from them. Not only have they lost the innocence they might have come in with, they have lost the ability to remember what came before and feel happy about it. Everything is coverered and affected by the pain and suffering of the unbearable present and reality of war.