Most of Robert Frost's poetry contains images and metaphors from nature, but even Frost once said of himself that he is not a "nature poet." He says in almost all of his poems there is "a man."
It seems Frost likes to use nature as the avenue to compare human struggles with a force that is constant. He is not, in fact, from the "romantic" period in American Lit, which explored nature as a avenue to the discovery of beauty.
Instead, Frost wrote in the 20th century - "Modern Lit" - his use of nature is his attempt to take something familiar and traditional, but to use it as a point of comparison for complex human emotions - to show the way man meets with real challenges but can take a baisic and natural approach to choices and answers.