Rhyming couplets give the stanzas of this dialogue a neat, tied-together sound, emphasizing the logical structure of the arguments. Things sound more convincing (and definitely more memorable) when they're bound together with rhyme. Take one of the soul's complaints deep in stanza 3:
And all my care itself employs;
That to preserve which me destroys (25-26)
The couplet rhyme of "employs" and "destroys" underlines their logical connection. Everything the body does destroys the soul.
Alliteration and Assonance
Marvel also uses alliteration like salt: sparingly, to enhance the flavor. Two punchy examples, using D- and B-words, occur in the first stanza, underlining the surprise of the soul's paradoxes:
Deaf with the drumming of an ear (6)
and emphasizing the discomfort of its prison:
bolts of bones (3)
The body gets into the game too, hitting the M's and L's in "has made me live to let me die" (18) and the H's in "hatred's hidden ulcer" (36). It's just making sure we realize how wicked that soul is.
Assonance, or the repetition of similar vowel sounds, also leaps out of the line to grab our attention by the neck. "Green trees" (44) stacks up some balanced E's, while "blinded with an eye" (5) doubles the long I sound without even using two I's. At line 20, the body points up its vexation with a series of short, snippy I's: "ill spirit it possest" (20).