Stephen Crane once wrote a poem entitled "A Man Said to the Universe." The poem is as follows:
A man said to the universe:
“Sir, I exist!”
“However,” replied the universe,
“The fact has not created in me
A sense of obligation.”
The poem perfectly captures exactly how much nature doesn't care about mankind.
That indifference on nature's part is exactly what "To Build a Fire" focuses on. The man in the story isn't named, because his name isn't important. A point of the story is how nature can give and take from mankind at any time regardless of how well prepared and confident a person is. The man in the story thought he could handle the incoming weather and avoid any major disaster. Nature showed him otherwise, and didn't care in the slightest that he died. In fact, not even the man's dog cared all that much.
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.
I think the man represents all of mankind and mankind's relationship with nature. We think that we have nature figured out. We think that we can predict the weather, withstand hurricanes, beat out freak freezes, etc., but time and time again nature shows us exactly how powerless we are against it.
London's story about nature's indifference and Crane's poem are not the only two examples that I can give either of nature's indifference toward humans. Sara Teasdale wrote a poem called "There Will Come Soft Rains" about the same topic, and Ray Bradbury based a story of his by the same title on the topic. The closing line of the poem goes like this:
And Spring herself, when she woke at dawn
Would scarcely know that we were gone.