The mood of "To Build a Fire" changes slowly, as the man discovers how unprepared for the cold he really is. At first, the story is told with simple phrases, little excitement, and shows the man's unconcerned nature. He does not worry that it is not sunny, because that is normal so far North. In addition, he is so confident that he can reach the campsite by nightfall that he carries almost no supplies; just matches and his lunch. Even his early sense that the Yukon cold is worse than any he has experienced is not enough to sway him.
Later, as he falls into a spring, wetting his clothing, the mood turns darker, more frantic; the man tries to light another fire, fails, tries to keep calm and finally, as he runs out of matches and has no way to light a fire and thaw himself out, breaks into a desperate run:
He ran blindly, without intention, in fear such as he had never known in his life.... The running made him feel better. He did not shiver. Maybe, if he ran on, his feet would thaw out; and, anyway, if he ran far enough, he would reach camp and the boys.
(Quotes: London, "To Build a Fire," eNotes eText)
This fear becomes his final mistake. The story represents the man's descent from calm deliberation into panicked flight; his brief spots of hope are erased by his inability to to save himself. The simple descriptions of the extreme cold and the man's slow death of hypothermia create an almost claustrophobic mood, and certainly one of inevitability.