an older man. He enjoys the lie. Second, the word truth also suggests the idea of being true or faithful in a relationship. The Dark Lady swears that she is true to the speaker, and the speaker pretends to believe her. However, he knows it is really a lie. He knows that she has other lovers. But it is better to pretend that she does not. If he thinks about her other lovers, after all, it would only bring him grief and sorrow and perhaps drive him mad with jealousy.
In the second quatrain we find that the Dark Lady is pretending as much as the speaker. She pretends that the speaker is young even though she knows that he is old, that his "days are past the best" (meaning his best days are past or over, line 6). Thus, both the speaker and the Dark Lady suppress or avoid the truth.
In the first two lines of the sestet (lines 9-10), the speaker asks two questions: Why does the Dark Lady claim or pretend to be true or faithful? And why does the speaker claim or pretend to be young? In the next two lines the speaker then answers those questions. In answer to the first question, the speaker states that "seeming" or pretended trust is the best way to behave or act in a relationship ("love's best habit"). If one does not have trust -- or at least pretend to have trust -- the relationship would not go well. The couple would fight. In answer to the second question, the speaker asserts that older people do not like to be reminded or told that they are old. The speaker would rather not think about his age at all.