Adams’ own thoughts on this issue can be found at the very end of the excerpt where he is recounting a conversation he had with Calhoun. There, he attributes the Southerners’ defense of slavery to their pride.
Adams says that the Southerners like slavery because they are vain. He says that having slavery in their territory allows them to lord it over other humans and treat them like dogs. It gives them a sense that they are better than the people of the North who do not have the ability to rule over other people in such an extreme way.
Thus, Adams believes that serious character flaws underlie the Southern defense of slavery.