Jim is an adult—and an adult who has become a whole lot like a parent to Huck throughout their adventures, protecting him and taking care of him (and later, of Tom as well) much as a parent would. And the behavior that he wants from Huck, when he wants anything at all, is prosocial in the extreme (an apology, to take the most famous example, for playing a trick on him in the fog; not much of an ask, it seems, unless you stop to consider that it’s a slave asking a white boy to acknowledge that he was in the wrong). Tom, on the other hand, is a peer. And his demands are far closer to the anti-social side of the scale. Is it so surprising, then, that Huck sides with his old mate?