You first see this in the second line “and sorry I could not travel both” implying that he is fearful of what he will miss out on if he takes the other road. This is also seen again in the third stanza “Oh, I keep the first for another day! Yet knowing how way lead on to way, I doubt if I should ever come back.” The speaker at first is optimistic about coming back to this point, then begins to doubt himself he will ever be able to find his way back. Comparing this to a choice he is hopefully he will be able to make this choice again but then realizes the regret he will have if he makes a choice either way