The famous opening lines of "The Road Not Taken" introduce readers to the choice the speaker faces, which will become the main focus of the poem: two roads diverge, and the speaker, unable to travel both, must choose between them. It's important to notice that, right from the start in line 2, the speaker reveals a sense of sorrow at having to choose between the two roads: he or she is "sorry" that choosing one road means missing out on the other. The speaker's struggle sets up one of the poem's main themes—the role of choice and uncertainty in life. It also reveals something important about the speaker's attitude towards the role of choice in life: his or her sense of regret that one is often forced to choose, and that choosing one thing means not choosing another.
The speaker's regret lingers through the rest of the poem, so that, even after he or she has made a decision, it is difficult not to wonder about what would have been had he or she chosen the other road. One of the core ironies of the poem is that it doesn't actually matter which road the speaker chooses, since both roads would leave him or her with a feeling of regret about what he or she might have missed out on. The poem's title also speaks to this dilemma directly, not only signaling that the focus of the poem is the road not taken, but even implying that there will always be a road not taken, and with it an unshakable feeling of regret over what one might have missed. Frost himself even indicated at one point that he may have modeled the speaker in this poem after an acquaintance of his named Edward Thomas, whom he described as "a person who, whichever road he went, would be sorry he didn't go the other."
In light of the choice presented in the poem's first lines, the most obvious guiding question for the poem may at first seem to be, "Which road will the speaker choose?" But if one keeps in mind that the speaker will be stuck with a feeling of regret no matter which road he or she chooses, the guiding question then becomes, "How will the speaker deal with his or her feeling of regret at having been forced, by the demands of life, to choose one road rather than the other?"