Good question! In a story like this one, where there's barely any action at all, it's a good idea to pay special attention to passages in which a character notices something unusual about himself. It will reveal the heart of the story—the theme, or the central message.

Whenever a character looks in a mirror or looks at a photo and realizes that he is somehow not himself, you want to try to understand how that character has recently changed, or how he has an appearance that doesn't match up with his real self, or how that character is somehow two people at once.

Nick glances into the mirror about halfway through the story, as he's swept up in his evening of talking and drinking with his friend:

His face looked strange. He smiled at the face in the mirror and it grinned back at him. He winked at it and went on. It was not his face but it didn't make any difference.

What does it mean that he's not himself, and that it doesn't even matter? We want to take a close look at recent changes or struggles within the character: here, how Nick nearly got married to Marjorie, but it didn't work out. That is, he nearly became a different person, or a different version of himself, by becoming a married man. But he didn't. And at this point, he's not sure if he wants to try to get Marjorie back, or if he should just let it go and become a kind of permanent bachelor, which would involve lots more evenings like this one—drinking and talking with his guy friends, being kind of macho but lonely and unfulfilled. But he doesn't want to become a drunkard.

When Nick looks in the mirror, then, and sees himself as a sort of grinning, winking, odd stranger, we understand that he's a man in a state of flux. He's not the person he was before meeting Marjorie, but he's not a married man, either; he doesn't recognize himself as even having a solid, single identity.

Importantly, though, Nick asserts that the fact that he's not himself makes no difference. It doesn't matter, he thinks, that he hasn't yet become whoever he's going to be. There's time, he's young, and he's got plenty of options in life. Check out some of the last lines from the story:

Outside now the Marge business was no longer so tragic. It was not even very important. The wind blew everything like that away.

You can interpret Nick's lack of a cohesive identity and his subsequent dismissal of its importance as evidence of Hemingway's tendency to portray the inconstant nature of everything in human life—even the pessimistic idea that because nothing lasts, nothing matters.