The speaker in Shakespeare’s Sonnet 138 makes the odd confession that when his lady love tells him that she is truthful, he supposedly believes her, even though he knows she is lying. Of course, he means that he pretends to believe her, but in fact he knows he cannot believe her, because he knows she is lying.
But he has some lying going on as well. He wants to make her think he is unsophisticated like a young man. So he pretends to believe her lies, in order to get her to believe his pretense at being younger than he is.
In the second quatrain, the speaker sums up the lying and falsifying on both parts: he knows that she knows he is not a young man in his prime, so he admits that his pretense is in vain. She does not really believe he is young, anymore than he believes she is a faithful lover. They both simply suppress the truth for the sake of their silly game.
In the third quatrain, the speaker rationalizes their deceptions and makes the ludicrous claim that “love’s best habit is in seeming trust.” The speaker knows better than this; he is a mature man who surely must realize that such “trust” is not trust at all. These lovers cannot possibly trust each other: they each know the other is lying.
The couplet offers little by way of assuaging the situation; it merely reveals that the relationship is based on a sexual relationship: “I lie with her and she with me.” The speaker is playing on the word “lie”; he has made it clear that both lovers “lie” to each other, so now when he says they lie “with” each other, he is referring to their sexual relationship: lying in bed together.
They are flattered by this arrangement, and flattery is not the basis of a serious relationship, but the speaker confesses that that is the kind of relationship they have. So what can they do?
Other articles on Shakespeare: Who is Shakespeare?
Sonnet Commentaries: Sonnet 1, Sonnet 2, Sonnet 3, Sonnet 4, Sonnet 5, Sonnet 6, Sonnet 7, Sonnet 8, Sonnet 9, Sonnet 10, Sonnet 11, Sonnet 12, Sonnet 13, Sonnet 14, Sonnet 15, Sonnet 18, Sonnet 19, Sonnet 116, Sonnet 126, Sonnet 130