Look for a moment at the title of this story: He Knew He Was Right. There are a couple of interesting ways of understanding that title. It seems to refer to Louis's suspicion about his wife secretly having the Colonel as her boyfriend, which he feels confident about. At the end of his life though, his wife asks for his blessing, as a sign that he knows she did not break their marital vows. The ironic tension here is that Louis must not believe she is sleeping with him. Rather, he knows that his young wife has secretly feelings for other men, and on that front, he is absolutely correct.
That would make the novel a story about jealousy, which it is, but another way of framing Louis's relationship to jealousy is to say that perhaps jealousy represents naïvete. That seems to be the case, since all the lovers in this book are fairly young. So, Louis and Emily are a newlywed couple, and Emily has started seeing another person (which is enough to create serious conflict in any relationship). But, perhaps the newlyweds should have used Emily's interests in the Colonel as an opportunity to celebrate honesty and trust. For Louis though, it's only an issue of jealousy. But, surely every couple has worked through something like what Louis and Emily fail to work through, so there is a maturity issue.
That doesn't mean that any of the characters are inherently wrong or immature, but it does serve as a reminder that the passions of youth are serious, and that young lovers feel emotions very deeply.
Also, Emily's tragic frustration serves as a good reminder that even without resorting to sex, a person can easily betray their spouse by spending too much time with someone they really like, instead of sacrificing those interests and investing in the marriage.
