The speaker of "Mutability" cannot help but show his sadness over the transience of beloved things in the world, even though he knows that change creates harmony in nature. Because this harmony is achieved through processes like death and destruction, it is tainted with sadness and melancholy. What’s more, you have to be a certain kind of person to appreciate the bittersweet character of mutability. If you’re greedy or a criminal, then you’re probably just riding the rollercoaster of fortune and ignoring everything else around you. But a calm, stable, attentive person will naturally feel a twinge of sadness when he or she considers how, as The Flaming Lips put it rather poetically: "Everyone you know, some day, will die."
For Wordsworth, "melancholy" is a grander and more praiseworthy emotion than mere "sadness." Melancholy takes into account the state of the whole of the external world, while sadness is self-centered.
The music of dissolution is a consolation prize for its terrible effects on humans and their surroundings. Music helps us cope with an all-too-appropriate sadness.