The ending of the story proves that the dog's instincts were superior to the man's reasoning. The man freezes to death in the snow, having made a complete debacle of his journey across the treacherous frozen land. The dog stays with him for a short time and then just goes trotting off to their common destination, where he knows he will find shelter, warmth and food.
Jack London explains that this is not just any dog but a dog not far removed from his wolf ancestors who had survived in this forbidding land for countless thousands of years. The dog has become domesticated, but it still has the instincts of its ancestors.
At the man's heels trotted a dog, a big native husky, the proper wolf-dog, grey-coated and without any visible or temperamental difference from its brother, the wild wolf....Its instinct told it a truer tale than was told to the man by the man's judgment....The dog did not know anything about thermometers. Possibly in its brain there was no sharp consciousness of a condition of very cold such as was in the man's brain. But the brute had its instinct. It experienced a vague but menacing apprehension that subdued it and made it slink along at the man's heels....
Throughout the story, the author takes great liberties in interpreting the dog's thoughts, feelings, instincts, and motives. Jack London was noted for this kind of anthropomorphism--"the attribution of human characteristics or behavior to a god, animal, or object." He was such a good writer, and he writes about his subject with such knowledge and authority, that the reader accepts it. London does the same thing in greater depth in his popular novel The Call of the Wild, in which the dogs seem to be more important characters than the humans. In "To Build a Fire" the reader can't help feeling that the dog is a better character in many respects than his cruel, greedy, selfish, and ignorant human master.
The trouble with him was that he was without imagination. He was quick and alert in the things of life, but only in the things, and not in their significances. Fifty degrees below zero meant eighty odd degrees of frost. Such a fact impressed him as being cold and uncomfortable, and that was all. It did not lead him to meditate upon his frailty as a creature of temperature, and upon man's frailty in general, able only to live within certain narrow limits of heat and cold; and from there on it did not lead him to the conjectural field of immortality and man's place in the universe.
Jack London needed a second character for his story, albeit an animal. If the man were completely alone in the blinding white wilderness, the reader would be confined to a description of the Chechaquo's stream of consciousness. And in this case, the human character is not really very interesting. There would be no letup, no variation in the narrative. Although man and dog cannot communicate, the reader can at least appreciate the differences between their points of view. The dog would like to be able to tell the man that they shouldn't be out here in this intense cold. But the man already knows he made a bad mistake in making this journey, especially without a human companion. He had been warned not to travel alone, and not to travel at all when the temperature was as low as this.
The inclusion of the dog as a character in the story enables the author to describe the man after he has frozen to death. The dog shows the same indifference to the death of this incompetent man as the vast, indifferent universe itself.
A little longer it delayed, howling under the stars that leaped and danced and shone brightly in the cold sky. Then it turned and trotted up the trail in the direction of the camp it knew, where were the other food-providers and fire-providers.