When my love swears that she is made of truth I do believe her, though I know she lies, That she might think me some untutor'd youth, Unlearned in the world's false subtleties. Thus vainly thinking that she thinks me young, Although she knows my days are past the best, Simply I credit her false speaking tongue: On both sides thus is simple truth suppress'd. But wherefore says she not she is unjust? And wherefore say not I that I am old? O, love's best habit is in seeming trust, And age in love loves not to have years told: Therefore I lie with her and she with me, And in our faults by lies we flatter'd be.
This poem has a speaker who says he knows his love is lying to him but he believes her anyway. Or at least he pretends to believe her so she will think he is young and naive and not as old as he really is. And she apparently pretends to believe this of him, although she knows he is not young. Both parties know the other is lying, and they also know the other knows of both sets of falsehoods. They tolerate each other's lies because they wish to have their own lies tolerated. So that's what they do and are flattered and seemingly happy with the situation. This is just another good example of why people tell lies, and how we can accept lies as truth even when we know they're not. People choose to believe what they will, for whatever reasons they will. Our truth is what we choose it to be, just like the two lovers in this poem. Our own versions of the truth do not depend on actual factuality necessarily; sometimes we all choose to believe something different. It may be because we need to, like in Life of Pi, or it may be for far less noble reasons, like in this sonnet where it's born of vanity and ego and a desire to be flattered. For whatever reasons, we all choose what "truth" we will believe.