One of the central themes of this poem is that every person is, at some point, torn between what they want and what others expect from them.
The speaker of the poem is clearly someone with responsibilities; busyness, for him, is so routine that even his horse thinks it is strange when he makes an unscheduled or atypical stop. It is a very dark night, and the speaker is arrested in his progress through the forest by the beauty of the scene around him: the "woods fill[ing] up with snow," the complete solitude (there is no "farmhouse near"), the sound of "easy wind and downy flake," and the perfect, unbroken darkness. This image is quite tranquil, and it seems to have a soothing effect on the speaker as well, given the language he uses to describe it.
He acknowledges that "The woods are lovely, dark and deep," emphasizing the beauty of the scene first. Because "dark and deep" describes the forest's loveliness, we can understand that the reason he finds it so beautiful is precisely because of that perfect darkness and the way the wood seems to go on and on, allowing him to forget -- momentarily -- the other responsibilities he has. Finally, however, he decides that he cannot stay in this beautiful place, though he might wish to, because he has "promises to keep." The speaker chooses his obligation to others over his own preference to stay in this peaceful place. Not everyone would necessarily make the same choice, but the poem does emphasize that it is a choice.