In "The Things They Carried," Tim O'Brien discusses many items that the men carried in the Vietnam War. Some of the things are physical, like photographs, candy, and grenades, and some of them are mental, like dreams and fears.
As the brutal war changes the men, the meaning of the things they carry also begins to change. Most of the men are shocked at how horrible the war is and how quickly people die for no reason. They go from being naïve young boys to hardened men who distance themselves from their lives back home and the objects that remind them of it.
For example, recall how Lieutenant Jimmy Cross carries the photo of Martha, the girl he loves back home. He also carries letters she writes to him. These objects remind him of what he felt like being with her, and he spends a lot of time daydreaming about her. Sometimes his daydreams of her distract him from what is going on around him. This happens when Ted Lavender dies, and afterward, Jimmy becomes frustrated and angry. He begins to understand that Martha belongs to a different world. He works hard to "shut down the daydreams" because he realizes,
This was not Mount Sebastian, it was another world, where there were no pretty poems or midterm exams, a place where men died because of carelessness and gross stupidity. Kiowa was right. Boom-down, and you were dead, never partly dead.
Now that Jimmy has really seen how easily people die in this war, he knows that he can no longer be distracted by items that remind him of the past. The things he had that reminded him of Martha went from being a nice connection to his home to a horrible reminder that he lives in a different place now and can no longer have such a simple, pretty life. He might not have to carry around the letters anymore, but he does have to carry the guilt he feels over Lavender's death. He carries it "like a stone in his stomach for the rest of the war."