"The Road Not Taken" by Robert Frost allows readers to contemplate the decisions they make. Some decisions are life-changing and others are more a matter of preference and do not change lives. However, for some, all decisions are difficult to make and there is always a possibility that, regardless of the choice made, the decision may turn out to be the wrong decision. Therefore, the main theme is about making choices.
In his poem, Frost is lamenting or complaining about this problem, not as it relates to himself but with regard to how others perceive (observe) it. The narrator would like to travel on both paths and is disappointed that he has to choose at all. He is conflicted or at odds with himself as he looks along one path and then chooses the other, observing that "it was grassy and wanted wear." However, he admits that there is probably little difference, them being "about the same." And so the theme is extended to include the effects of choices.
In this instance, the narrator worries more about the choice he does not make rather than making the most of the one he has chosen. Understanding that the theme is about choices also allows the reader to contemplate or consider expectations. The narrator can be happy about having taken "the one less traveled" and be confident that "that has made all the difference" or he can be troubled by his choice and the "difference" that that will make to his future in a potentially negative or undesirable way. Therefore the theme also includes elements relating to the consequences of risk-taking or avoiding risk.