Deception
In Sonnet 138, the speaker and his lover deceive each other. The lover swears that she is faithful to him. The speaker likes to pretend that he is younger than he is. In fact, they are both lying to each other: “simple truth [is] suppressed.” The speaker chooses to “believe” his lover’s lies because it allows him to pretend to be younger than he is: only an inexperienced boy would choose to believe his lover is faithful to him when it is so obvious that she is not. Letting himself be deceived is flattering because he can act like a young man again.
Age
The speaker of the poem is self-conscious about his age. He knows that his “days are the past the best.” Yet he recognizes that not everyone wants to admit how old they are when it comes to romantic matters: “age in love loves not to have years told.” This is why he never says openly to his lover that he is “old.” Instead, he falls for his lover’s lies because he thinks this shows him to be an “untutored youth” who is clueless about how the world works. She plays along, and this makes him happy: “in our faults by lies we flattered be.”
Thinking versus Knowing
The sonnet shows that what we know and what we think or believe sometimes diverges. For example, the speaker’s lover “swears that she is made of truth.” In response, the speaker says: “I do believe her, though I know she lies.” This paradoxical statement shows that knowing the truth does not always affect what we choose to believe. Similarly, there is a contrast between what the lover thinks and knows. The speaker pretends to be younger than he is and his lover pretends to fall for it in order to make him feel better: “Thus vainly thinking that she thinks me young, / Although she knows my days are past the best.” In other words, allowing himself to think the thing his lover pretends to think strokes his ego. It is of no importance what his lover actually “knows,” i.e., that he is old.
Faults
Both of the characters have faults. The lover, associated with the "Dark Lady" of the latter half of Shakespeare's sonnets, is not very beautiful. She is also dishonest and cheats on the speaker. The speaker's fault is that he is old. He knows that old age does not suit lovers and he is self-conscious about this. Yet the magic of lies is that lies allow them to "flattered" by these faults. If they pretend not to see them, they do not have to acknowledge them.
The Best Kind of Love
At first glance, Sonnet 138 reveals a quite dysfunctional relationship. Both lovers are lying to each other: he about his age and she about her fidelity. Worse, they do not confront each other about their lies. Instead, they choose to believe them. Yet the sonnet argues that this is the best kind of love: “Oh, love’s best habit is in seeming trust.” The use of “Oh” at the beginning of the line gives this statement the appearance of an aphorism (an expression of a widely accepted truth). In fact, it is a rather unique approach to describing what the best kind of love is. For the speaker, “seeming trust” is better than actual trust. To trust in how things seem (rather than how you know them to be) allows you to make yourself and your lover feel better. The lover, for example, pretends to believe that the speaker is younger than he is. Seeing his lover pretend to believe this makes him happy and allows him to pretend not to see her infidelities. This trust in appearances is, for the speaker, “love’s best habit,” meaning both that it is the best thing lovers do and that it is the best clothing (”habit”) or appearance for love to wear. Lying suits love best. In fact, it is “lying” that allows the lovers to “lie” or sleep together.
