Why are the small things so important to characters?

In this post-apocalyptic world, the small things are all the boy and his father have left. Humanity and the world are dead or on its last gasping breath. A can of Coke becomes something precious. The boy insists on sharing it and thus the can becomes a communion between the boy and the man. Even the small sporadic conversations between the boy and the man also become special. McCarthy is a master at saying so much with so little.