This is a story about injury and isolation. The characters who suffer from physical injury also suffer from emotional injury. Though an establishment has been created to help these men heal, they necessarily suffer alone, in isolation, having lost something crucial that once connected them to the world.
The narrator is isolated from the others in a number of ways. The fact that he is American is arguably the primary factor in his isolation, yet this is in addition to his injury. The loss of his confidence, his idealism and health also isolate him psychologically.
The major suffers from the loss of his young wife, isolating him from his friendship with the narrator and rendering him bitter and hopeless.
The injuries to the bodies of these men can be seen as symbolic, suggesting that it is the human condition to suffer alone though we may celebrate together. This theme connects many Hemingway stories and novels.