At the end of the poem, in the couplet, the speaker concludes that, for the reasons he has given (in lines 9-12), he and the Dark Lady have decided to lie to one another. They have decided that this game they play is in their best interest. However, the speaker adds one more word in explanation: flattery. He explains that hiding their faults (his old age and her infidelity) with lies makes them both feel better. They are "flattered" in pretending that the lies are true. It is flattery. It is vanity. But it makes them happy.