The speaker of Frost's poem stops by the woods because the snow falling against the dark night sky is so beautiful. The speaker has people to see and places to be, but nevertheless, he is so taken by the stillness and solitude of the scene as the snow piles up that he stops, spellbound. It is simply lovely to watch the flakes of snow fall like the softest of feathers (they are "downy") against the dark sky. As the speaker says, it is a night of:

easy wind and downy flake.

The woods are lovely, dark and deep

In the first stanza the speaker tells why he is stopping by the woods. It is "To watch his woods fill up with snow." It is a cold night but apparently not too cold for the speaker to stop for a few minutes to look at a beautiful sight. I think all of us have done this at one time or another, though most of us were not riding in a horse-drawn sleigh. We are on a motor trip and see a beautiful view. There may even be a marked turn-off where motorists can park and enjoy the view. There are plenty of such places, for example, around the Grand Canyon and probably in every national park. Frost was a nature lover. He not only enjoyed looking at beautiful natural scenery, but he seems to have drawn inspiration for some of his poems directly from nature, as was also done by famous English poets like William Wordsworth and John Keats. 

"Stopping by Woods on a Snowy Evening" reads like a tribute to the beauty of nature. If we get anything out of the poem at all, we get the feeling of being there on a cold, dark, silent night watching the "downy" flakes slowly descending like white feathers and settling on the trees. It would seem that these woods would have to be evergreens because the speaker says it is "the darkest evening of the year," which would make it December 21st or 22nd. At that time of year all the deciduous trees in the region would be bare or nearly bare of dead leaves, and this would not be a pretty sight. But evergreens with their widespread branches covered with snow are always so aesthetically appealing that they are often depicted on Christmas cards. 

If the trees are evergreens, that would explain why the speaker seems apprehensive about being seen by the owner of the woods. That other man sees the woods as a commercial investment. If he found the speaker stopping there and looking at his woods, it wouldn't occur to him that someone was just enjoying the beauty of nature--especially on a cold night with the snow falling. Christmas is just a few days off, isn't it? Most people in New Hampshire probably don't go to some Christmas tree lot to buy their trees for the holidays. They probably go out and chop one down. The owner would undoubtedly think that the speaker--Frost himself presumably--was thinking of taking one of those trees home with him.

The plot of "Stopping by Woods on a Snowy Evening" by Robert Frost is quite simple. The speaker rides in his horse-drawn sleigh or carriage through the snow to the edge of the woods where he stops near a farmhouse and watches the snow fall for a time. While he is patient and unmoving, his horse is impatient and restless, jangling his harness bells as if to tell the speaker that he is more than ready to leave. That's it. Nothing more really happens. 

The bigger question is the one you ask: why did he stop here tonight? This stopping place is clearly familiar to him, as he knows who owns this land. It is a spot close to civilization yet away from it, and the speaker obviously finds something about the darkness of the woods compelling. The lake is frozen, the snow has been falling, and the night is dark so the whiteness should shine even brighter; instead he is drawn to the shadowy darkness of the woods. Most of us would think all the glistening whiteness is beautiful; however, the only thing the speaker refers to as beautiful is the woods while they are inescapably, inseparably covered in glistening whiteness. To him, 

[t]he woods are lovely, dark, and deep....

The horse is impatient to leave, but the speaker regrets not being able to stay--or at least he regrets having to go back to his "real" world. The final stanza of the poem gives us the best hint about why the speaker of this poem stopped here tonight to look at the woods.

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. 

Though he does not tell us explicitly why he stops, the speaker clearly wishes he could do something more than fulfill the promises he has made in life. He regrets that he cannot explore or get lost or do something other than what he has committed to do, the thing that is so wearisome that he has to repeat the fact that he has miles to go before he can sleep. It will be a long time before he can disentangle himself from the things he is bound to and the woods represent something about which he is wistful; however, he will fulfill his obligations and take the memory of those "lovely, dark, and deep" woods with him.

Robert Frost answers this question himself. He does so in the first stanza. He stops

To watch his woods fill up with snow.

Frost was extremely sensitive to the beauty of nature. He is similar to William Wordsworth in this respect. Frost's "Stopping By Woods on a Snowy Evening" might be compared with Wordsworth's sonnet "The World is Too Much With Us." Wordsworth says

Getting and spending, we lay waste our powers;—

Little we see in Nature that is ours;

In "Stopping by Woods on a Snowy Evening," why does the man get back to his journey in the last stanza?

I spent a summer at the Bread Loaf School of English several years ago. Robert Frost's farmhouse is part of the campus, and we had an annual "Robert Frost picnic" on the grounds. It is located on top of Bread Loaf mountain, just outside the town of Ripton, Vermont.

If you ever visit, you'll understand why Frost would stop in the middle of a snowstorm to watch a field be covered with snow. It is absolutely beautiful. A river (or large creek; can't remember) runs parallel to the road up the mountain, and the rocks and the water are mesmerizing.

As beautiful as the scenery is, reality calls him to his business. He doesn't tell us what he has to do, but he takes care of his responsibilities.

In "Stopping by Woods on a Snowy Evening," why does the man get back to his journey in the last stanza?

In the poem "Stopping by Woods on a Snowy Evening" the speaker is brought back to himself during his night time ride by the gentle shaking of the harness bells.  As he ponders the peaceful scene of quietly falling snow in a serene wood, perhaps he is enjoying a moment of peace after a busy day, when suddenly the soft tinkling of the bells on the horse's harness draw his attention back to the task at hand--that of making his way home on a snowy evening.

In "Stopping by Woods on a Snowy Evening," why does the man get back to his journey in the last stanza?

Well, I'd say he starts to come out of his dream state in the third stanza, when the horse shakes its harness and makes the bells jingle. That sound rouses the man a bit, enough for the thoughts to slip in.

And what are the thoughts? Work and duty. Specifically, when he comes back to himself, he remembers all the promises he's made, and all he must do before he goes to bed.

Greg