In his hurry to get the fire going, what did the man forget in "To Build a Fire" by Jack London?

After the man traveling alone with only a dog breaks
through a wet spot as he continues his trek following his lunch, he wets himself halfway
to the knees.  Now, he must build a second fire to dry his feet or he will be lost to
frostbite and death.  Struggling with frozen fingers to untie the laces, the man
realizes that he must cut them with his knife.

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But before he could cut the strings it happened. 
It was his own fault, or, rather, his mistake.  He should not have built the fire under
the spruce tree.

Because the
man "lacked imagination" and has no instinct of things, he does not anticipate that the
fire under the spruce tree laden with snow will cause the snow to melt and fall into his
fire, quenching the flames. Each time he has pulled a twig from under this tree, he has
slightly moved branches and has created "an imperceptible agitation" that is "sufficient
to bring about disaster."  This act is the man's nemesis.  Because he has not traveled
with another man as the old man advised, there is no one to help him. Now he hears "his
own sentence of death" in an uncaring nature of which he has been ignorant, unlike the
dog whose instincts have told him it was too cold to be out.