This story can be related to the famous first line of Tolstoy's novel, Anna Karenina.
“Happy families are all alike; every unhappy family is unhappy in its own way.”
Hemingway's story explores the isolation of several individuals who have been injured in war. Though some of the men band together as a result of their injuries, two men find themselves isolated. Their isolation is not of one kind, however. The isolation of each individual is distinct. They are all isolated in their own way.
The American narrator experiences his own special isolation as does the major who befriends him.
This notion underpins the entire story as the men find themselves suffering from more than physical pain of injury. They suffer also from the emotional impact of isolation, responding in various ways to the various modes of isolation they suffer.
We can read this aspect of the story as relating to the human condition in general. Those who are unwell are unwell in their own way and in such a way as to become isolated.
The most poignant example of isolation comes in the character of the major. He, like the narrator, is struggling to undergo the rehabilitation treatments on the machines and he becomes friendly with the narrator.
Despite the things these two men share in common, which are significant, the major suffers from the unexpected loss of his wife. This cuts him off from the narrator, as the major suffers from his own special emotional injury, which the narrator cannot understand.