Anse gets worked up about Jewel's desire to bring his horse on the journey, thinking it shows disrespect for Addie. Anse cannot believe that Jewel would feel okay "prancing along on a durn circus animal" while the rest of the family rode together on the wagon with Addie in her coffin.
Anse convinces himself that he has the authority to deem the behavior of others too self-interested and disrespectful of the family's sense of duty. This is especially ironic in the context of the Bundrens in particular, as Jewel has already been revealed to feel intense emotion for his mother and her legacy, while Anse focuses principally on himself.
Darl laughs and Anse wonders how and why his son is able to laugh with his dead mother in her coffin laying at his feet. The wagon passes Tull's lane just as Jewel and his horse catch up with the family's wagon. Darl continues to laugh.
Darl's laughter emphasizes his ability to detach from the situation at hand—sitting in the wagon with his mother's coffin—and to have perspective on it. This can perhaps be seen as Darl's self-interested tendency that keeps him at a distance from the romanticized ideal of fulfilling a heroic familial duty. Darl knows that they journey they are setting out on is ridiculous.