In the poem human beings are seen as always in a hurry. They have places to go and things to do. It is not the norm to stop in the woods. Yet nature holds a certain allure. A line by line look at the poem can be useful as a means of illustration:
Whose woods these are I think I know.
His house is in the village though;
These are not the narrator's woods. he is clearly an outsider passing through. In fact, these woods are uninhabited. Traditionally, a empty wood at night would be a place of fear, but here the speaker seems not to feel threatened. In fact, he pauses only to consider that as these woods are not his own and whether or not he should stop on someone else's property, remarking:
He will not see me stopping here
To watch his woods fill up with snow.
It is not that the owner of the woods will or will not mind. He will not see, so he will not know. there is a bit of an illicitness about the act of stopping, of man disturbing nature with his very presence, and the horse is the one who is the "witness" to the act:
My little horse must think it queer
To stop without a farmhouse near
The horse is symbolic of society that looks upon the act of stopping as inconsistent with the norm, but the pull of nature is too strong for the narrator to resist, even as it brings with is a sense of possible danger, not only from being caught, but from the elements themselves:
Between the woods and frozen lake
The darkest evening of the year.
Nature presents dangers in the cold. The man could become sick. A frozen lake can be a deadly trap. Darkness in the woods holds the possibility of countless dangers. Nature can be either a friend or an enemy, and in realism nature is usually an obstacle. But Frost was not always in line with the rest of the realists. The horse, however, seems to think that the man is tempting fate:
He gives his harness bells a shake
To ask if there is some mistake.
He urges the man to remember that society is waiting, that time is passing by and that he should be moving on, but nature has its own commentary and corresponding pull to stay a moment longer:
The only other sound's the sweep
Of easy wind and downy flake.
In fact, nature seems soothing here with adjectives such as easy (simplicity in nature as opposed to the complexities of life in an urbanized world) and downy (reminiscent of a feather bed, perhaps)
The woods are lovely, dark and deep.
They call to him, it seems. Nature is urging him to take a pause, but he can only comply for a moment before he returns to the civilized world:
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.
Nature may not be burdened by responsibility, but man is. He must attend to his business before he can sleep (the first sleep) and he has much that he must do before he dies (the second sleep) whereas nature, while it does consist of life cycles, has no concept of death or responsibility. It simply exists. It is not bothered or impressed by the man, but he is with it.