Sonnet 138

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 untutored youth,
Unlearnèd 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 supressed.
But wherefore says she not she is unjust?
And wherefore say not I that I am old?
Oh, 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 flattered by.

Note: Sonnet 138 is about the lies we tell ourselves to remain happy. He has a mistress who swears to her honesty, and he believes her in spite of knowing its all lies. This way she thinks him young and naive. In this way they fool each other: he is not young and she is not honest. But why do they choose to do this? Because its easier to just flatter each other, and live a happy, flattering lie.