This poem can be read as a narrative, or it can be interpreted figuratively (as a metaphor for all of life's decisions). My question is that, what is the main message that applies to both interpretations?
Whether you read the poem literally or metaphorically, it can be viewed as a poem about the missed opportunities and a sense of regret one experiences as a result of that.
Literally speaking, the poem is about a traveler who is standing in the woods, looking at two roads which diverge. He feels sad that he has to pick one path and no matter how long he looks down one path, he cannot predict what it holds. Even when he chooses the path which he calls "less traveled," he cannot suppress that feeling of regret arising from the fact that he will never be able to discover what the other path offered. Would it make his journey more eventful? Would he feel more fulfilled or not? He will never find out. And that knowledge evokes a sense of sadness.
Metaphorically speaking, the poem is about making an important choice in life. Once that choice is made, one's life will be altered for good. There is no going back. The speaker feels disappointed that choosing one path will always prevent him from discovering what the other path offers. He is aware that once the choice is made, he cannot change it. He has to stick with the choice he made.
Therefore, whether we read this poem literally or metaphorically, a sense of regret resulting from the knowledge that we have to choose between the opportunities we are offered ties both interpretations together.