In setting the tone the author writes, “They carried the sky. The whole atmosphere, they carried it, the humidity, the monsoons, the stink of fungus and decay, all of it, they carried gravity.” This quote is both dramatic and it sets the tone just by its references to dankness and weight. Metaphorically and physically, the soldiers carried the weight of the world on their shoulders and within themselves in this story of the Vietnam War, a war that many feel was futile.
War is hell, but that's not the half of it, because war is also mystery and terror and adventure and courage and discovery and holiness and pity and despair and longing and love. War is nasty; war is fun. War is thrilling; war is drudgery. War makes you a man; war makes you dead.
In this quote, again by O’Brien, the author writes of the juxtaposition of emotions, actions, and ultimate ending when faced with all that is war. This quote encompasses all that war is to the soldier who experiences the ultimate highs and lows of both inner conflict and of battle itself. The author uses this tone to inflict upon the reader what a soldier sees, feels, and lives during a battle. Although the soldier lives at the end of this book, he often feels dead within himself due to the aftermath of living through the war.
O'Brien often chooses drama (dialogue) and objective narration because he wants to provoke thought in his readers. He wants to avoid subjective melodrama and he also wants to avoid overt moralizing by the narrator. He wants the book to seem realistic, not preachy or propagandistic.
If you are referring to only the first chapter of the novel where O'Brien literally discusses the long list of things the soldiers carry, both literal and emotional, then you could surmise that the long list and the long chapter serve to weigh down the reader with the overwhelming weight of everything these soldiers had to deal with. He would use an emotionless tone in regards to the equipment of war and a more dramatic or emotional tone when mentioning something personal like the fact that the men carry their memories of home or that they must carry their personal fears.