Sonnet 138 - When My Love Swears That She Is Made Of Truth by William Shakespeare

 In the sonnet, the narrator describes his beloved's proclamation of herself as "truth" i.e. she is an embodiment of truth. Our clever narrator further says that he pretends to believe that indeed she is truth although he knows that it is completely false. She might be thinking that he is some inexperienced youth who is unaware of the falseness in the world and in particular, dealings of love. 

The beloved is also clever as she also tries to make him believe that he is still young. He knows that he is no longer a youth but an old man who is past his prime age. On both sides, pretensions is made but both knows the truth, but the plain truth is never spoken.

Then the narrator asks the rhetorical question, why shouldn't she credit her just nature and he, admit his old age? Then he answers that in love, the "best habit" is of pretension of trust between the partners. Even though the lovers know about their partners shortcomings, they live by them with ease. This is how love grows with age. Nobody is perfect in his/her traits. The narrator wants to say that the only way to love is to love the bright side of one's partner and ignore the dull/ugly parts.

The narrator sums all by saying that their mature understanding of each other's faults binds them together and that's how they live with each other happily ignoring each other's flaws.