The doctor's wife has found food in a supermarket cellar and has gone through a time of blindness herself in the darkness of the place until she found matches. Once more we see her face a moral dilemma of keeping her knowledge to herself or sharing with the starving people who have been unable to locate that door to survival. Once she is out safely with a supply to bring back to her little group, she stops and finding herself lost, sinks to the ground in exhaustion and near-despair when a dog comes up to her and licks her tears, gently consoling her.
I would consider the dog as a metaphor or symbol perhaps for hope as the woman reaches her limits. Or maybe as a sign of the courage that comes at that last hour just when all looks lost. Somehow, however, I find it a metaphor for the past. "This too shall pass," is one of my own personal favorites, and I'd like to think that looking back on what we've come to know (as the dog remembers his puppy behavior) makes us strong and experienced enough to face the present.
As a side note, the Simon & Garfunkle tune of the lyrics above keeps playing in my head as I read this. When I gave it the attention it seemed to require, I realized that what we don't see in the story is the rest of the world as it played out in this disaster. And I think with horror of a zoo where the animals are no longer being fed.