What is the Central Metaphor in Robert Frost's "The Most of It?"

Taken literally, the "he" of the poem wishes that the universe and nature are there to provide him some kind of emotional and spiritual support. He wants more than just the return of his own voice (in echo). This is an allusion to the myth of Narcissus and Echo. Hera cursed Echo, making Echo only able to repeat the words of others. Narcissus is famous for falling in love with his own reflection in a pool of water. Frost seems to be using these allusions to show how there is a tendency for a person to be selfish in expecting more from nature (more than an echo). The selfish man might get his own reflection or echo in return, but this is selfish thinking if he actually thinks this is nature giving love back particularly to him. This critique is summed up as, "the world doesn't revolve around you." 

It is therefore more logical to simply "make the most of it," to make the most of what nature offers us. If we see something beautiful or stirring in nature, it is not necessarily nature answering our cries or prayers. (This is the Naturalist interpretation of nature). It is simply nature being nature, a spatio-temporal field of events we can experience and interpret in our own ways. A thunderstorm, a rainbow, or a calming breeze are all just "there." These are not responses to a single person's cries. But these are all there for each and every person to experience in whichever way they choose. 

You could suggest that in addition to the literal meaning, nature could also be a metaphor for society. In other words, the "he" of the poem wants more out of life than he believes life is giving him. He selfishly thinks "he kept the universe alone," as if to say he is the center of the universe. He knows that the deer is just doing its own thing but he likes to think that this event is there for him to experience. Think of this metaphorically in terms of this man and other people in his life. He wants to think that all others exist to be parts of his life; they exist to fill the world of his existence. But if he was more open-minded and selfless, he would understand that each person ("each buck") and each event in life is not necessarily in existence for his own personal gain or enjoyment. He has to realize that he is only one part of this large interactive (ecological) community. He should consider that the buck (metaphorically, another person) has a story, desires, dreams just as much as he does. In this metaphor, nature can also be thought of as a social world and the buck can represent any other person in the man's life.