The speaker explains the reason for not being able to remain for long in the woods in the last three lines of the final stanza:
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.
No matter how much the speaker is transfixed by the woods which are "lovely, dark and deep," he is awakened from his reverie by a sense of duty towards himself and others. Although we do not know what promises the speaker has to keep or where he has to return, we do know that he has to leave this pleasant sight of "the darkest evening of the year" because he feels he must fulfill his obligations.
There has been a lot of debate about the potential meaning of "the woods" and "promises" that have to be kept. Some critics believe that the woods may embody the world of fantasy or even death, which the speaker pines for. This is because the depiction of the winter scene in the poem evokes images of darkness and immensity ("the woods are lovely, dark and deep") and a sense of tranquility, both symbolic of death. "Promises" are thought to refer to the speaker's dedication to embrace life and all it offers, including the promises he made to himself and others. Although death may seem rather enthralling, his life missions are still not accomplished. As implied by the last three lines of the poem, there are still promises to be kept, journeys to be undertaken, battles to be fought. Until all of this is finalized, this brief, but soothing and captivating respite in the snow-laden woods should remain as a pleasant memory which will hopefully stimulate the speaker to succeed in whatever comes next.