Shakespeare, of course, abandoned the traditional Petrarchan sonnet and developed one in which the three quatrains examine a subject from differing perspectives. In his complete sonnets, an older man addresses the first 126 to a young man, while the others are addressed to or are about a rather sensual woman who is the older man's mistress. Often these latter sonnets have the love triangle of poet-friend-mistress.
In Sonnet 138 the theme is the consummation with his mistress. Here are the three perspectives presented in the quatrains:
1. Although the speaker knows that his mistress lies to him, he believes her so that she will think him like the young man, an "untutor'd youth" who is refreshingly naive.
2. By his pretense, the speaker gives his mistress credibility which also suppresses the truth that his "best days are past."
3. In suppressing the truth, they exhibit a trust in each other.
The couplet, then, concludes that in the consummation of their love as they lie together, the poet and his mistress are both flattered and satisfied.
In a previous sonnet, Sonnet 130, the poet has pointed out the shortcomings of this mistress, but yet declared his love for her with some humor and more than a touch of reality. Again, in Sonnet 138, the poet emphasizes that his love for his mistress is grounded in reality as opposed to the Platonic idealizations of love found often in Petrarchan sonnets.