Why does the man have to make a fire before he eats lunch?

The man's hands and fingers are frozen and numb, not to mention the fact that he is unable to move his mouth due to the ice-muzzle created by spitting tobacco which had frozen in his beard.

He unbuttoned his jacket and shirt and drew forth his lunch. The action consumed no more than a quarter of a minute, yet in that brief moment the numbness laid hold of the exposed fingers. He did not put the mitten on, but, instead, struck the fingers a dozen sharp smashes against his leg. Then he sat down on a snow-covered log to eat. The sting that followed upon the striking of his fingers against his leg ceased so quickly that he was startled. He had had no chance to take a bite of biscuit. He struck the fingers repeatedly and returned them to the mitten, baring the other hand for the purpose of eating. He tried to take a mouthful, but the ice-muzzle prevented. He had forgotten to build a fire and thaw out.