I think that the issue of choice plays such a prominent role in the thematic meaning of the poem reflects how a part of the poem's message is to stress the idea how one must view consciousness as filled with challenges and uncertainty.  Consider how the exposition of the poem stresses this idea for the challenge within choice: "Two road diverged in a yellow wood,/ And sorry I could not travel both."  At the outset of the poem, there is a clear establishing in the challenges in life.  The poem's message from its inception is to make clear that there will be choices that face the individual, what Frost called an example of "the pleasure of ulteriority."  It is choice that defines what it means to be a human being.  These choices are filled with challenges and the biggest challenge is the uncertainty into which the individual is poised. consider the idea of how the divergence of roads challenges the speaker.  Simply put, the challenge is the choose and there is no external reality that will make that choice for him.  With the ending as being one in which the choice of the speaker "has made all the difference," the message of the poem becomes clear that there are challenges and their uncertainty is what defines the need to make such choices.  It is here where one could see how the message of Frost's poem is to bring this condition out to the individual, illuminating it so that one sees choice as a part of individual consciousness and not lose sight of the importance that choice has on the individual regardless of its uncertainty and lack of totality.