A story, however short, is seldom written to demonstrate a thesis in the same way as an academic essay, so there may be a variety of plausible answers to this, based on the reaction of individual readers and what they think is the story's most important theme. With that in mind, here is my view:
A life lived purely for selfish pleasure is ultimately hollow and boring, deracinating the one who lives it and cutting him or her off from anything that might give life meaning.
This sounds dangerously like a moral, which is why Hemingway never says any such thing, but he is constantly demonstrating it. The lives of the two protagonists are superficially glamorous and exciting. As the girl says, they "look at things and try new drinks." The flatness of their conversation, however, reveals that they are sick of everything they do. Even the revelation that they are having a child, rather than being the traditional source of joy and celebration of a new life, merely entails a clinical discussion on...
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