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.