Emasculation and wounded masculinity
Jake's physical wound is the novel's central fact, but the theme of damaged manhood runs through every male character. Cohn performs masculinity through aggression and romance. Mike performs it through bravado he cannot back up. Jake cannot perform it at all. Hemingway forces the reader to watch men fail at being the men they think they should be.
The Lost Generation's aimlessness
The characters drink, travel, and argue because they have no clear purpose. The war destroyed the frameworks, religious, social, and personal, that would have given their lives direction. The novel does not offer a replacement. It just shows the emptiness those frameworks left behind.
Love as something that cannot be fixed
Jake and Brett love each other genuinely, and it does not matter. Their love cannot become what either of them needs it to be. The novel refuses the idea that love is enough to solve anything. It shows love as a source of ongoing pain rather than salvation.
Authenticity versus performance
Romero is the novel's standard for authenticity. He does real work with real skill and does not pretend. Almost everyone else in the novel is performing: performing toughness, performing indifference, performing happiness. Hemingway rewards Romero with the reader's admiration and punishes the performers with each other's contempt.
Nature as temporary relief
The Spanish countryside and the fishing trip offer the characters, and the reader, a brief escape from the social performance of Paris and Pamplona. Nature does not fix anything, but it provides a space where the noise stops. Hemingway uses those scenes to show what the characters are missing and cannot hold onto.