The Question and Answer section for To Build a Fire is a great resource to ask questions, find answers, and discuss the novel.
The man is meant to represent the folly (ignorance) of all men in the face of mother nature, "He was a newcomer in the land, a chechaquo, and this was his first winter." The man with his ego represents the human misconception that we can somehow...
We learn the man is inexperienced.... something that will give him trouble later.
The man spits into the wind and sees his spittle freeze before it hits the ground.