It is the beauty of the pristine snow that blankets the field near a dusted woods and frozen lake that distracts the speaker of Frost's poem from his obligations.
Certainly, the speaker of "Stopping by Woods on a Snowy Evening" experiences a transcendental moment as he observes,
The only other sound's the sweep
Of easy wind and downy flake
While he watches the snow fall, the speaker feels one with nature in the peace and tranquility of the lovely pastoral scene before him. As opposed to the mundane affairs to which he must attend, the speaker experiences what Emerson calls "the perpetual presence of the sublime." Truly, the speaker is the Emersonian lover of nature whose inward, as well as his outward, senses are moved by the beauty.
And, because his entire being is touched by his experience of the nature's peace and breath-taking beauty, it is difficult for him to shake himself from this transfixion. Instead, it is his horse who must shake his harness bells to remind the speaker that he must "harness" both his distracting thoughts and delight in nature and return to the mundane matters and obligations of his life: "But I have promises to keep."