The speaker of "The Road Not Taken" by Robert Frost clearly has some regrets, or at least some wistfulness, about his decision to take the "road less traveled by."
Our first evidence of that is the title. Notice that he is not celebrating the path he chose but rather regretting, or wondering about, the road that was not taken.
The second piece of evidence is that both roads were just about the same. This poem is often celebrated as a choice to take a path which few others have traveled as some kind of individualistic and bold move; in fact, the speaker tells us both roads were "just as fair" and says
Though as for that the passing there
Had worn them really about the same,
And both that morning equally lay
In leaves no step had trodden black.
He could have chosen either path and been content with it, but he had to choose one.
The third evidence of the speaker's wistfulness or regret is that he decides to keep "the first for another day!" Unfortunately, though, he knows it is unlikely that he will ever get back there, knowing how life moves on and things get in the way of our best intentions. Often taking one path in life (making one decision) necessarily eliminates other choices forever. His regret is seen most in the final stanza:
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.
He will sigh, presumably with some regret, about this decision many, many years from now. He seems to understand that some choices in life are irrevocable, and this was one of them.
He is still torn at the end of the poem, when he kind of divides himself by putting a dash between "I." He talks about "all the difference" his choice has made, and often that is celebrated as a triumph. In fact, however, there is no implication of either positive or negative effects from that choice; it is a neutral comment. That decision "has made all the difference," has simply determined the course of his life, whatever that is.
The thing is, neither path was a bad choice. Both were "fair" and equal in their attributes; however, he had to choose one, and that is the course his life took. It is an understandable wistfulness, as we have all had to make a similar choice and then wondered what our lives might have been like if we had made the other choice. Generally there is some regret or wistfulness, even if we are perfectly content with where we are now.