Who is the speaker, and what is the situation?
While many have interpreted Robert Frost's "The Road Not Taken" as an extended metaphor for life and its choices, Frost himself made light of such interpretations.
On more than one occasion the poet claimed that this poem was about his friend Edward Thomas, a man inclined to indecisiveness out of a strong—and, as Frost thought, amusing—habit of dwelling on the irrevocability of decisions. [Enotes]
So, the speaker can be Edward Thomas, a friend of Frost's with whom he took walks; as they walked, Thomas would see one path and feel that he might wish to go down it, then another would appear favorable and he would become indecisive and want to change his mind. After the two men finally went down one path, Thomas would wonder if the one not chosen would have been better and dwell upon the possibility.
Regardless of what the intended meaning of the poem is, there is merit in the discussion of either interpretation of the situation of having to choose one path over others as both are reflective of Thomas's--and that of many others--preoccupation with the finality that decisions hold. Especially significant are the lines,
And both that morning equally lay
In leaves no step had trodden black.
Oh, I kept the first for another day!
Yet knowing how way leads on to way
I doubted if I should ever come back.
So often people make choices in life, then wonder if they had gone a different direction regarding personal relations, or careers, or places to live, etc. what their lives would have been like instead. There is no question that many decisions are, indeed, irrevocable, as Thomas worried. Frost himself remarked that “The Road Not Taken” is “a tricky poem, very tricky.”