In this chapter, the men on patrol in Vietnam have just witnessed the death of Curt Lemon, who steps on a round of live ammunition. In an act of symbolic retribution, Rat Kiley kills a baby water buffalo. The inconsistency of these acts, and the nearness of life and death, makes the narrator think "proximity to death brings with it a corresponding proximity to life" (page 77). He states that the nearness of death during a battle makes soldiers aware of "the immense pleasure of aliveness" (page 77). After a firefight, he is intensely aware of everything around him—the soil and the grass, and he says that he becomes aware almost of being out of his skin and wanting to be the best human being he can be. As he says, "In the midst of evil, you want to be a good man" (page 77). In other words, being that close to death makes him appreciate every little object and act of decency that makes life better. He describes studying the river and the sunset on the night before a battle, as he attempts to drink in the essence of life before it might end for him. The proximity of death makes him acutely appreciate the components of life.
This irony exists for the soldiers because when they see how quickly death can come for anyone of them, they also see just how precious life is for them. Note how earlier when Ted Lavender is killed how it changes the men. Lt. Cross, in particular, has a greater appreciation for life, mainly the lives of his men.
Remember too the soldiers are under incredible amounts of stress. This stress, a long with the fact that each man is responsible, directly and indirectly, for the lives of his platoon members, helps bond the men together like never before.
This is but one of the many ironic statements O'Brien uses in this work.