As a reporter, David has a special understanding for the subtle ways that individuals are shaped by history. For Jack, the Great Depression was a major defining feature of his personality. It wasn't just history; it literally shaped his perception of reality for years to come. Even when money wasn't nearly as pressing of an issue, Jack still felt compelled to work jobs he hated, because of honor.
But that leaves David in a difficult emotional position, because David wants to be honorable too, but by following his calling, he has had to abandon his own culture and its assumptions of masculinity.
As he attempts to reconcile his life of writing, reporting, travel, and adventure to his brother, he can't help but feel guilty, as if his brother is secretly a better person than him, because he did what was best for his family (he thinks). But at the end of the day, the novel itself is the difference.
Because the novel itself exists (and because the stories are known by critics to be somewhat autobiographical) we can understand David's frustration with a layer of knowledge that he (as a character) does not have access to. We can see the beauty of the story, depicted by a skillful author, and that is literally an answer to David's frustration. The answer is that every person has their own destiny and they are only obligated to fulfill it—whether they obey their culture along the way is beside the point entirely. In other words, people have to be who they feel they are.
