Summary
Because of the beautiful things he sees in the bower, says the speaker, he now knows that nature's beauty will always be there for those wise enough to find it, even in places that don't seem very grand on the surface—it's all about whether you remain open to nature's gifts. Not only that, he says, but it can actually be a good thing to not have access to everything you want. These small deprivations can prompt the soul to work harder, creating rich imaginative and empathetic moments like the one the speaker has just experienced. He then addresses Charles again, and describes a rook flying overhead in the dusk. He says that he blessed the rook as it became more distant from him, knowing that it would soon fly over Charles himself. The speaker says that Charles certainly appreciated the sight of the bird, because, for him, a sound created by nature or by a living thing can never be unpleasant or dissonant.
Analysis
We can think of this last segment of the poem as being divided into three parts. The first is a kind of moral—a takeaway from the experiences the speaker has had. The second is also a moral, but a slightly more complicated and enigmatic one. The third is perhaps another moral, but this one, not explicitly stated, is instead wrapped up in a declaration of love and admiration.
The first moral, introduced with the phrase "Henceforth I shall know," is a reference to the speaker's ability to eventually find beauty in the bower. The bower, he has realized, isn't as obviously beautiful or lively as the sights his friends hiked off to see. Yet, once his bitterness had given way to openness, he found that his initial dismissal was unfair. In other words, the world is bursting with beauty, and there is neither a "plot so narrow" or a "waste so vacant" that you can't find a bit of beauty. The problem isn't boring, ugly, or lifeless places: it's people failing to appreciate the natural world. This concern fits into the Romantic movement as a whole, in which Coleridge played a leading part: the Romantics were concerned about industrialization and urbanization, and urged others to appreciate and defend the natural world.
The arrival of the second moral actually helps the first one feel less preachy or didactic. Coleridge introduces it through casual musing—in the middle of a line, the speaker nonchalantly continues, "and sometimes." This stream-of-consciousness moralizing makes us feel that the speaker is just figuring things out, taking us along for the ride rather than talking down to us. This second moral is slightly stranger than the first. The speaker asserts that it is good to "contemplate/With lively joy the joys we cannot share" (that is, to happily imagine nice things going on even in your absence). That seems uncontroversial, but he doesn't simply say that imagination is a good way to cope with loneliness—he actually says that it's good to be excluded or deprived of pleasures precisely because it prompts you to use your imagination. This must mean that imagination can be even better than experience—perhaps because it's a way to develop empathy and better understand your friends. Indeed, imagining Charles, even though the two aren't together, seems to have made the speaker feel close to him in a new and different way.
And, in fact, at this point the speaker turns his attention back to Charles. He's praised his friend throughout the poem, but here it more or less becomes a full-blown love poem. The speaker praises Charles as an exemplar of the morals and values he's just articulated. Charles, he explains, is never resentful or closed-off to the natural world. He already has a deep appreciation for it, even in its humbler manifestations. Meanwhile, the natural world itself here literally connects the speaker to his friend. As a bird flies overhead, he imagines it crossing from himself to Charles, allowing the two to look at the same thing even though they're far from one another. Earlier in the poem, when the speaker was at his most isolated and upset, he saw nature as a beautiful but impenetrable expanse separating him from his loved ones. Now, though, he sees nature not as the thing dividing him and Charles, but as the thing connecting them—and while nature itself hasn't changed, his appreciation of it has.
