Sonnet 138 presents a candid psychological study of the mistress that reveals many of her hypocrisies. Certainly she is still very much the poet's mistress, but the poet is under no illusions about hercharacter: "When my love swears that she is made of truth, / I do believe her, though I know she lies." He accepts without protest her "false-speaking tongue" and expects nothing better of her. Cynically, he too deceives and is comforted by knowing that he is no longer fooled by the woman's charade of fidelity to him, nor she by how young and simpleminded he presents himself to be.
In a relationship without affection or trust, the two lovers agree to a relationship based on mutual deception. Both agree never to voice the truth about just how much their relationship is built on never-spoken truths: "But wherefore says she not she is unjust? / And wherefore say not I that I am old?" Note that the sentence construction in these two lines is identical, similar to how both the poet and the woman identically feign lying when each knows that the other person knows the truth.
The main theme of the concluding two lines is lust, but it is treated with a wry humor. The poet is content to support the woman's lies because he is flattered that she thinks him young — even though he knows that she is well aware of just how old he is. On the other hand, he does not challenge her pledges of faithfulness — even though she knows that he is aware of her infidelity. Neither is disposed to unveil the other's defects. Ultimately the poet and the woman remain together for two reasons, the first being their sexual relationship, the second that they are obviously comfortable with each other's lying. Both of these reasons are indicated by the pun on the word "lie," meaning either "to have sex with" or "to deceive": "Therefore I lie with her and she with me, / And in our faults by lies we flattered be."