Why doesn't the dog trust the man in "To Build a Fire"?
Other than the fact the he sensed that the man was willing to kill him so he could warm up his hands, not much :) But I think the real question, the one that London is suggesting, is "Why didn't the man trust the dog?" The dog has "natural" instincts that allow him to survive. It knows instinctively that the man will not survive in the conditions; it knows that must live with and respect nature if you are going to survive.
This also might be considered a reply to the romantic view of nature. Romantics viewed nature as a source of energy and consolation for us; other authors, such as Melville, took a more sober view of our relationship with nature. For London, nature is just there; deal with it and respect it, and all will be well. Ignore or challenge it, and beware the consequences.
The dog knew this instinctively; sadly, the man did not.
First, London was one of the naturalist writers. Their writing focuses on the conflict man vs. nature--and nature wins. Even though the dog is domesticated to an extent, it is still part of nature. In the beginning of the story, London mentions something about the dog not wanting to be out or knowing they shouldn't be out. So, we know anything natural will win against man's stupidity. Also, the dog could sense something wrong with the man's behavior and tone of voice. Animals may not understand the exact words, but, as the Dog Whisperer tells people, they pick up on our vibes. I know you've heard the saying "animals can sense fear." The man was reaching desperation--that does something to your tone and body language. Some people also believe it does something to your aura or energy.