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.