The mood or tone of Robert Frost's poem "The Road Not Taken" can perhaps be best described by the word nostalgia. It means looking back on the past with sentimental emotions. The poem can be profitably compared with Shakespeare's famous sonnet #30 which begins with the following lines.
When to the sessions of sweet, silent thought
I summon up remembrance of things past
After a certain age--say forty-five or thereabouts--it is common for people to look back on their lives and reflect about what they could have done differently, or should have done differently. It is appropriate that Frost uses the two roads diverging in the woods as a symbol or metaphor, because no doubt as a young man he was actually taking long walks by himself and meditating on his biggest problem of that period, which is most people's biggest problem when they are young. The roads represent possible career choices. He knew he wanted to write poetry, but he knew he also had to earn a living. And poets generally make very little money, if they earn anything at all.
Your whole duty as a writer is to please and satisfy yourself, and the true writer always plays to an audience of one. Start sniffing the air, or glancing at the Trend Machine, and you are as good as dead, although you may make a nice living.
E. B. White, The Elements of Style
The speaker in the poem, who is undoubtedly Frost himself, knows he doesn't really want to change and knows he is too old to change now even if he wanted to. But he can't help wondering whether he made the right decision, whether he really had any choice in the matter, or whether everyone's fate is predetermined. Life is a mystery when we are young, and instead of getting less mysterious as we grow older, it gets more and more mysterious.
The poem is inconclusive. All that is resolved is that he had to make a choice and he chose to lead a simple, rural life not unlike that of Henry David Thoreau, and to devote most of his time to creative writing. This inevitably meant writing about nature, since there was little else to write about. Life is largely a matter of making choices at what William James, the distinguished American philosopher and psychologist, brother of the great fiction writer Henry James, called "crossroads situations."