In "The Road Not Taken," why does the narrator choose one path but wish he could have taken both?
The narrator is at a point in his life where he has two choices. He does not want to skip over experiences in his life, since he can't know if they will be once-in-a-lifetime, but he knows that he must make a specific choice. Thinking about both (looking down the path as far as he can), he thinks that they are equally valid, but that one is somewhat more unusual than the other (grassy and wanted wear). His choice is to take the "road less traveled" since it might offer an experience that he cannot duplicate with the road that more people take. In the last stanza, the narrator muses:
I shall be telling this with a sigh
Somewhere ages and ages hence:
Two roads diverged in a wood, and I—
I took the one less traveled by,
And that has made all the difference.
(Frost, "The Road Not Taken," bartleby.com)
He foresees that he might regret his decision, but he knows that he has made it to the best of his ability. He makes the choice that is not often made, and knows that his future life will be changed forever ("made all the difference") because of that choice. It's not that the choice is positive or negative, but that it is one that he will probably not get to repeat, and so it affects everything that will happen to him in the future.
It is written in tetrameter and while iambic is used, the first two words are spondaic to help the reader visualize that there are two roads. Frost also uses anapestic -- (in a yel)
At first glance the narrator thinks that one of the paths seems to be better "having the better claim because it was grassy and wanted wear: but afterwards realizes that "though as for that, the passing there had worn them really about the same".
He was sorry he couldn't travel both partly, because he wanted to go down the best path and therefore had to make a decision; what would happen if he didn't make the correct decision...he might miss out on something wonderful.