In an alternate universe in which World War I never happened, Jake would be a total stud. It’s like what they say about James Bond: women want him, men want to be him.
Unfortunately, in our world (and that of Hemingway’s novel), World War I most certainly did happen, and it left an ugly scar upon everyone it affected. Jake is no exception—his scars are not only mental, they’re physical:
[Georgette] looked up to be kissed. She touched me with one hand and I put her hand away. "Never mind."
"What’s the matter? You sick?"
"Everybody’s sick. I’m sick too." (3.4)
He has a frequently mentioned but somewhat mysterious war wound that renders him impotent, which explodes the image of him as a hero. Even though women do want him, they can’t have him; while men admire and respect him, they certainly don’t want to be him.
So what is Jake’s deal? He’s a hero who’s not a hero, a man who can’t perform his… er, manly duties, an American in self-imposed exile from America. What does all this add up to?
The perfect embodiment of the Lost Generation, mentioned in the epigraph by Gertrude Stein. Jake represents the aimlessness and perpetual dissatisfaction of the post-World War I era; he was permanently marked by the war, and he’s just one of the thousands and thousands of young men damaged by it. He may try to keep a cool-as-a-cucumber front, but when he lets his guard down he's spooked:
It is awfully easy to be hard-boiled about everything in the daytime, but at night it is another thing. (4.25)
Jake is an emblem of an entire generation damaged by the trauma of a war of global scale, the likes of which had never been seen before.
But Jake isn’t as straightforward as he seems. He’s both a typical and an atypical representative of the Lost Generation. We quickly discover that he is different than the novel’s other main characters; he possesses genuine passion and aficion, which allow him to gain some distance from the dissolute and cynical world he dwells in. This shines through in his love of bull-fighting, fishing, and the natural world.
It is this difference that allows him (and us, his readers) to see through the fragile relationships and disingenuous attitudes of the people around him. Jake has a sense of something greater—he is a somewhat confused Catholic, but a Catholic nonetheless, and is the only character that has anything resembling real religious faith.
He’s also one of two characters in the novel that’s actually active and productive at work—Jake is a successful newspaper man, while Cohn is a failed writer and Mike and Brett appear to have no careers to speak of. Bill is the other productive character (we know that he’s a published author), but we only see him on vacation here. All of these differences make Jake the ideal narrator; he’s both outsider and insider, and he allows us to view the world he lives in from its center, but with some degree of objectivity.