Frost's poem is one that is near bittersweet in its reminiscent quality. As the rider pauses to reflect, we experience the beauty of the wintertime imagery made by snow, the jingle of the harness bells, and the other senses that are engaged by Frost's creative use of the language.
The last lines make this poem close to bittersweet:
The woods are lovely, dark, and deep,
but I have promises to keep,
and miles to go before I sleep,
and miles to go before I sleep.
It seems as though the rider would like to pause a while longer to enjoy the wintry scene before him, yet he knows he must move on to fulfill his obligations. This may be the only experience that is almost painful in the poem. The rest conveys a tone of contentedness and restful peace.
Frost's "Stopping By Woods on a Snowy Evening" conjures a tone of quiet reflection and wintry reminiscence. When the rider first stops, he knows he is trespassing, but knows the owner's house is in the village. Winter imagery is obvious, of course, but the reader is left questioning about "the darkest evening of the year." The reader reflects, as the rider is reflecting on the scene and ruminating about "darkness" and "winter." The quiet reflection and feeling that there's some work to be done arrives full force in the last stanza. With "But I have promises to keep" and the twice repeated "and miles to go before I sleep" we are reflecting on a job at hand and left wondering what these promises are that keep the rider out so late and in such wintry weather. The feelings are not of pain or sadness, but more of a meditative appreciation and realization that there's still much to be done that day (or figuratively in the rider's life).