In "Stopping by Woods on a Snowy Evening," there is significance in the simple incident. The speaker has seen the woods fill up with snow many time, but he decides to stop this snowy evening, the darkest evening of the year. Why would anyone stop on the darkest evening of the year? He stops with miles to go before he sleeps. Why would anyone stop on the darkest evening of the year with miles to go? 

Perhaps, the speaker is tired of traveling. Perhaps, he is wishing he could stop and forever stay in his neighbor's woods. Perhaps, he is contemplating suicide. Perhaps, he does not want to go on:

The speaker emphasizes that he has no practical reason to stop, that he is stopping for the beauty of the scene only. However, in line 8, an element of darkness appears, which can indicate that all is not well. Because the speaker also emphasizes the cold with "frozen lake," readers begin to understand that the poem may not be a simple light-hearted celebration of nature.

Something urges the speaker on. He remembers that he has promises to keep. His sense of responsibility is awakened. He realizes that he has promises to keep, so onward he must go. He cannot stay in the lovely, dark woods on a snowy evening. Life must go on. 

Why does the speaker stop by snowy woods on the darkest evening of the year? Even the little horse thinks it is strange. The horse realizes that something is wrong. He shakes his harness bells. The horse is alert to the fact that his rider would not normally stop on the darkest evening of the year with miles to go. So the speaker moves on for he has promises to keep: 

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.

This does not mean the speaker is enthusiastic about having to move on--to keep going. Perhaps, the speaker would prefer a permanent rest in the lovely, dark woods. Perhaps the speaker is tired of living life. Still, he repeats that he has miles to go before he sleeps:

The repetition of this line as the conclusion to the poem indicates that the idea contained in it is highly significant. Although the speaker may literally have "miles to go," the line also functions as a metaphor. He has much life to live before he can "sleep" permanently in a "dark and deep" woods. These lines suggest that although death may at times be more attractive than life to the speaker, he is nevertheless determined to choose life. The tone of the lines, however, may also indicate that the speaker is resigned to life but not necessarily enthusiastic about it.