The last stanza is the protagonist's projection of what he thinks the future will hold. He imagines himself relating this day with a sigh to another, and letting them know that when he came to the fork in the road he took the road less traveled, and that made all the difference.

We must remember two things the author said, first it is the story of his friend, Edward Thomas, and second Frost described this poem as "tricky" (Grimes, 2006). Though the roads are described as being for all intents and purposes equal it is obvious they are not. The first road is "bent in the under growth" while the second is "grassy," "wanted wear" and "the better claim." The protagonist took the second road. In other words he took the easy way. The protagonist asserts that he would like to take both roads, and understands he will never have this chance again. Frost then moves the action to the future where the protagonist imagines telling the story with a heavy heart of how when he came to the fork in the road he took "the one less traveled by." This is of course a self-delusion, a repainting of the past for self aggrandizement.

On one level the poem can be seen...

On another level the poem can be interpreted as a reflection of the human condition. We are all faced with choices every day, and most of us follow the path of least resistance. Following this path can preclude us for following our dreams. Few of us when we get older want to admit that we didn't climb that mountain or grab that prize because the road was too rough so we embellish the difficulties of the road we did travel in order to maintain our self-esteem.
This is a poem about choices. How we choose to lead our lives and how we choose to remember those choices.

References

Grimes, L.S. (2006, November 13). Robert Frost's tricky poem: Analysis of the road not taken. Suite101.com. Retrieved September 18, 2010, from Suite101.com: http://www.suite101.com/content/robert-frost-s-tricky-poem-a8712

Frost, R. (1920). The road not taken. Mountain interval. New York: Henry Holt and Company, 75. In Bartleby.com. (1999). Retrieved Septmber 18, 2010, from: http://www.bartleby.com/119/1.html