"We want to live at any price; so we cannot burden ourselves with feelings which, though they might be ornamented enough in peace time, would be out of place here." Chp. 7 pg.139 The soldiers give up their day-dreaming and their memories of home for the sake of preserving their lives. How would such dreams and memories interfere with their desire to preserve their lives?
This question reminds me a lot like Tim O'Brien's "The Things They Carried." In that story, as is the case in wartime anywhere, closing a zipper, dreaming of home, a girlfriend or family member can make someone particularly vulnerable because when we daydream, we are caught up somewhere else beyond our physical location. We can often be surprised when someone speaks to us while daydreaming, not even aware that anyone is nearby. In trying to protect yourself and your fellow-soldiers, thinking of anything but the moment at hand could cost one life or many.
In many other points of the story, Paul talks about an almost animal-like instinct that saves his life and the lives of others. He speaks of a feeling (he cannot really describe it) that warns them to hit the ground just moments before a shell explodes nearby. In many ways, dwelling on memories of the past would distance these humans even further from the animal-instincts which allow them to survive.
It is part of Remarque's larger point about war: that it destroys the humanity of the boys who are called upon to fight it. The worst casualties of the war, he suggests throughout the book, are the survivors. This quote points to this horrific truth about war.
The brute realities of war that Paul and his fellow soldiers face are terrible, dreadful and shocking. As Paul reflects in this quote, the only function that memories of home or of a brighter existence can have is to burden you and act as a handicap. In order to stand the best chance of survival you have to live, breathe and act like a machine without any emotions or dreams. Anything that could distract you from the daily struggle of survival is something that is a danger.
The values of peacetime are often incompatible with the values of war. Thoughts about the happy past might depress a soldier in the midst of war as s/he thought about all the pleasures of the past. Depression would help no one survive during war. It might make the depressed person less able to concentrate on present tasks, thus endangering that person as well as the person's comrades.
War is a pragmatic institution; it is about two things and two things only: killing the enemy and preserving yourself. All other thoughts and emotions are detrimental to the mission at hand; hesitation will get you killed, as will guilt or sympathy. On the battlefield, fighting the enemy is your only goal, and your own life is the most important because every day you continue living is a day you continue to fight. While at war, or in the mindset of war, you must be a single-minded creature without empathy or passion, or you will be crushed under the boots of your enemy, who have no empathy for you.