In the first quatrain of the sonnet, the speaker pledges himself to the mistress, while he humbly refers to himself as "I that vex thee." It can be roughly paraphrased as: You have me, and me, and me again.
The second quatrain can be paraphrased thus: Since your will is large and spacious, won't you let me hide my will in yours? Especially since you are graciously accepting others, but not myself?
In the third quatrain, he likens the mistress to an ocean, which would be able to comfortably accommodate an additional quantity of water. Thus, he implicitly gives up the right to an exclusive relationship with the mistress.
There is some debate over the meaning of the final couplet; in her book The Art of Shakespeare's Sonnets, Helen Vendler supported the interpretation by G. B. Evans (Shakespeare's Sonnets, 1996) as "Let no unkind [persons] kill no fair beseechers."
Counting the contraction wilt as instance of the word will, this sonnet uses the word will a total of fourteen times. The word is also a pun on the name of the author, and as such, is also used in Sonnet 134 and Sonnet 136.
Since "will" is a colloquial term for both the male and female genitalia, the poem can also be understood sexually in any number of ways.
In the 1609 Quarto edition of Sonnets, all capitalized instances of the word Will appear in italics.