Why does the man build an emergency fire?

In "To Build a Fire," the man builds an emergency fire after he steps into a small spring under the snow and gets wet halfway to his knees.
Jack London's short story exemplifies the theory of naturalism which holds that man is subject to natural forces beyond his control. Without the instincts of the dog that accompanies him, the somewhat inexperienced man looks for signs of a spring, but "at a place where there were no signs," his foot breaks through the ice under the snow, and he is wet halfway to the knees.

he would have to build a fire and dry out his footgear. This was imperative at that low temperature—he knew that much, and he turned aside to the bank, which he climbed.

The man also knows that there can be no failure in starting a fire; he must have heat soon if he does not want to suffer frostbite or death.

When it is seventy-five below zero, a man must not fail in his first attempt to build a fire—that is, if his feet are wet. If his feet are dry and he fails, he can run along the trail for half a mile and restore his circulation. But the circulation of wet and freezing feet cannot be restored when it is seventy-five below. No matter how fast he runs, the wet feet will freeze the harder.

Keenly aware of this danger, the man works carefully to start his fire. As the flame strengthens, the man adds twigs to it by reaching nearby. However, he has not paid close enough attention to where he has started the fire: "He should not have built the fire under the spruce tree." Instead, he should have started the fire in the open because there no misfortune can occur. For as the man pulls the twigs from under the tree, he "communicated a slight agitation to the tree—an imperceptible agitation." But it is enough to eventually cause one bough of the tree to unload the snow that sits on its thick branches. This snow hits a bough beneath it and a chain reaction occurs. Without warning the man is struck by snow; then, his fire is smothered with a blanket of it. "It was as though he had just heard his own sentence of death."