Unlike some of the other poems there is some dark humour in here. The scene is an older man with his lover who is a much younger woman. Even today there is still public prejudice to such a relationship. Whilst it may be unfair, the relationship to which the poem refers is indeed full of cynicism and is conducted with an air of foreboding. The poet is telling us there is a sort of balance between the two people, or rather; they are as bad as one another so everything is ok. Right? Well, no. We can tell that this temporary arrangement will end in tears.

When my love swears that she is made of truth,

The man is saying that the woman needs to be believed when she is making statements about her fidelity. She wants to be known as "pure truth". Apparently there is the suggestion of "maid of truth", perhaps Joan of Arc. As a French woman bent on the destruction of England, she would be a traitor.

I do believe her though I know she lies,

This line is taken in two parts. The first part has two meanings. "I do believe her" means "I kid myself that she is telling me the truth" and "I actually tell her I believe her." The second part "I know she lies" means that he knows in his heart that she is a liar. He believes because he likes what he hears. "I know she lies" also means that he knows she lies down with other men to have sex without telling him.

That she might think me some untutored youth,
Unlearned in the world's false subtleties.

These next two lines can be taken as one complete concept. Apparently one of the lies accepted by both parties is that the man is not really old. By believing her lie he exhibits one of the traits of youth - naive enough to believe anything. Youths, he says, know nothing of the world's subtle falsehoods and their place in every human relationship.

Thus vainly thinking that she thinks me young,
Although she knows my days are past the best,

This is a reiteration of the last two lines but its repetition makes the lie more obvious and shameful. She knows damn well he is old but the man bathes in the vanity that her lies falsely create.

Simply I credit her false-speaking tongue:

He accepts the lies. He puts up no opposition. He carries on cheerfully. "Credit" could also be "currency" or something that makes him richer. It would, of course, make the woman poorer to tell the lie. In "false-speaking tongue" there is a hint of "Eve" bearing the apple.

On both sides thus is simple truth suppressed:

This is a deadly line and quite cruel to both parties. She lies - he does not challenge the lie. One gets the impression that Shakespeare is here looking up from his own position and looking around at you and me and the whole world. "Simple truth" could also have a religious meaning so that they are both in sin and doomed to failure.

But wherefore not says she that she is unjust?
And wherefore not say I that I am old?

Shakespeare asks himself, and us, why they don't tell the truth when the truth is indeed simple. Why does he not say he is old and why doesn't she say that she is a whore? It is suggested here, and in other sonnets, that she sleeps with other men. "Unjust" is an oblique word to use but the meaning is clear.

Oh, love's best habit is in seeming trust,

He gives us the answer. Two people who are lovers pretend that they trust each other. They both lie because they want to make love and enjoy themselves. "Seeming" trust may mean "it seems, or looks like, like they trust each other." But in their hearts they do not.

And age in love, loves not to have years told:

He gives another reason. "Age in love" or an old man lusting after a young girl does not like to be reminded of his age.

Therefore I lie with her, and she with me,
And in our faults by lies we flattered be.

He rounds off the poem by playing with the word "lie" yet again. It means, of course, "to tell an untruth" and "to lie down and have sex". The "faults" are the lies they say to each other, but the fact they both "believe" them allows them to have sex. The acceptance of lies by both parties flatters their respective vanities.