Consider the symbolism of the woods. The speaker stops at a point where no houses are in sight. He sees woods in all directions. He is away from civilization. It is cold and dark, the darkest evening of the year. Other than the horse's bell, the only sounds are the wind and snowfall. The speaker/traveler is isolated, peering into the woods. He is contemplating and/or in the process of discovering something that he usually does not take the time to consider.
The woods are "lovely, dark and deep." Frost noted that with no comma after "dark" the implication is more optimistic. In other words, with this punctuation (or lack of it), the woods are lovely because they are dark and deep. If the comma comes after "dark" as well as "lovely" the three terms share importance. And this suggests that the woods are more mysterious or even foreboding. They are as much "dark" as they are "lovely" and "deep."
Whether or not you interpret the woods as mysteriously wondrous and/or mysteriously foreboding, the speaker is still peering into a landscape or a realm which he usually does not look into. Even the horse senses something is odd. The speaker takes the time to look at or ponder upon a scene or an idea that he usually would pass by without a thought. So, there is the sense that this is a metaphor comparing exploration to discovering new places, experiences, and ideas. But it is also a metaphor of thinking and contemplation. Peering into the woods is symbolic of deep thinking. And in either or both interpretations mentioned earlier, this is symbolic of deep thoughts of humanity's dark and wild possibilities as well as its potential loveliness lodged in these unexplored realms of the external world or in the mind.
The speaker would like to take more time in experiencing this deep thinking. But he has to get back to the social world. And he has miles to go before he can sleep and perhaps dream about such things. The dream itself is an interesting idea here because it arises from the depths of the subconscious. This is also a relatively unexplored realm of the mind, something that might be considered lovely, dark, and deep as well.