The main theme of this story is a perennial one in the works of Jack London: man versus nature. The hapless protagonist of "To Build a Fire" foolishly thinks that he can conquer nature, that he is physically and mentally strong enough to endure whatever the harsh, snowy wilderness can throw at him. But he's profoundly, and tragically, mistaken. In setting himself up as superior to nature, the man is treating the natural world as little more than an object, something to be used and exploited. He doesn't respect nature; he doesn't think it has a life of its own. This overweening arrogance is what leads him to embark upon a suicidal journey through the vast, icy wastelands.
It's notable that the dog he takes along with him on the journey has a better understanding of nature. As an animal, the dog leads a more natural existence, and this makes him sense the huge dangers involved in undertaking such a perilous trek through the snow. Unlike the man, the dog lives to fight another day. But that's because he was always attuned to nature, never in conflict with it. The dog's survival symbolizes the moral of the story: in the seemingly never-ending struggle between man and nature, nature will ultimately prevail.