"Sonnet 75" has some cool dialogue going on, so we're going to go ahead and encourage you to read it out loud with a friend. If you do, you'll realize that the speaker is really into alliteration; he just can't stop repeating sounds when it's his turn to speak.
Check it: we get the words "die" and "dust" (10), "Where," and "whenas" (13), "love" and "live" (14) almost back-to-back in the poem. Since this poem is partly about the poet's own awesomeness and immortalizing, poetizing skills, we think that he's going over the top in these last lines. It's like he wants us to say: "Hey, this dude knows how to alliterate." Check out those mad alliteration skills. He must be a serious poet—for realsies. Alliterative sound in this poem is pretty much a form of bragging rights for the speaker.
So, when you read this poem out loud, go ahead and emphasize those alliterative sounds. It's what the speaker would want, we promise.