The visible sign of the poet's preoccupation--the word is not too strong--is the recurrent image, particularly in his earlier work, of dark woods and trees, Often, as in the lyric with which we have begun, the world of the woods..., a world offering perfect quiet and solitude, exists side by side with the realization that there is also another world, a world of people and social obligations. Both worlds have claims on the poet. He stops by woods on this "darkest evening of the year" to watch them "fill up with snow," and lingers so long that his "little horse" shakes his harness bells "to ask if there is some mistake." The poet is put in mind of the "promises" he has to keep, of the miles he still must travel. We are not told, however, that the call of social responsibility proves stronger than the attraction of the woods, which are "lovely" as well as "dark and deep"; the poet and his horse have not moved on at the poem's end. The dichotomy of the poet's obligations both to the woods and to a world of "promises"--the latter filtering like a barely heard echo through the almost hypnotic state induced by the woods and falling snow-is what gives this poem its singular interest.... The artfulness of "Stopping by Woods" consists in the way the two worlds are established and balanced. The poet is aware that the woods by which he is stopping belong to someone in the village; they are owned by the world of men. But at the same time they are his, the poet's woods, too, by virtue of what they mean to him in terms of emotion and private signification.
. . . .
What appears to be "simple" is shown to be not really simple, what appears to be innocent not really innocent.... The poet is fascinated and lulled by the empty wastes of white and black. The repetition of "sleep" in the final two lines suggests that he may succumb to the influences that are at work. There is no reason to suppose that these influences are benignant. It is, after all, "the darkest evening of the year," and the poet is alone "between the woods and frozen lake." His one bond with the security and warmth of the "outer" world, the "little horse" who wants to be about his errand, is an unsure one. The ascription of "lovely" to this scene of desolate woods, effacing snow, and black night complicates rather than alleviates the mood when we consider how pervasive are the connotations of dangerous isolation and menacing death.
From "From Woods to Stars: A Pattern of Imagery in Robert Frost’s Poetry." South Atlantic Quarterly. Winter 1959.