And his dark secret loveDoes thy life destroy.
Here, alliteration links different words together: "love" with "life" and "dark" with "does" and "destroy." This is no coincidence. In fact, these words essentially
tell the poem in miniature. The sonic connection between "love" and "life" reflects how "love" has a major effect on "life" in the poem—in fact, "love," when "secret," can end "life" itself. The "dark" adjective, meanwhile, relates to the "invisible worm," with "does" relating to his action, and "destroy" describing the consequences of his actions. Darkness, the worm's illicit activities, and destruction, are all part of the same package.
The particular quality of the /d/ consonant is important too. In general, the poem sounds soft, its gentle, hushed tones creating a quiet atmosphere. The /d/ sound, by contrast, is a loud, voiced consonant. Here, the sudden alliteration gives the poem an air of violence. The heavy, thudding sound reflects the catastrophic effect that the worm's actions will have on the rose.