The poem itself neatly divides Wordsworth's relationship with and understanding of nature into two neat stages. When Wordsworth first came and contemplated the view that is before him now, he says that his appreciation and understanding of nature was very different. Note how he describes his reaction to the view in his youth:
...when like a roe
I bounded o'er the mountains, by the sides
Of the deep rivers, and the lonely streams,
Wherever nature led: more like a man
Flying from something that he dreads, than one
Who sought the thing he loved.
Wordsworth's youthful reaction to nature was all about movement and energy, not quiet contemplation. As he says, it was about emotion and passion. Nature was to him "an appetite." Now, however, he has matured and changed. The "aching joys" of nature have passed, but this is not something that makes Wordsworth sad, because the way that he contemplates nature now reflects his maturity and wisdom:
For I have learned
To look on nature, not as in the hour
Of thoughtless youth; but hearing oftentimes
The still, sad music of humanity,
Nor harsh nor grating, though of ample power
To chasten and subdue.
Now, when he contemplates nature, the speaker feels "a sense sublime" and is filled with the joy of "elevated thoughts." Nature now is about far more lofty thoughts and ideas than the passionate appreciation he had of it in his youth. Now, in his maturity, contemplating nature allows him to listen to the "still, sad music of humanity" which has the power to "chasten and subdue" us if we have ears to hear it.