Whilst I alone did call upon thy aid,
My verse alone had all thy gentle grace;
But now my gracious numbers are decay’d,
And my sick Muse doth give an other place.
I grant, sweet love, thy lovely argument
Deserves the travail of a worthier pen;
Yet what of thee thy poet doth invent
He robs thee of, and pays it thee again.
He lends thee virtue, and he stole that word
From thy behaviour; beauty doth he give,
And found it in thy cheek: he can afford
No praise to thee, but what in thee doth live.
Then thank him not for that which he doth say,
Since what he owes thee, thou thyself dost pay.
In plain English
Once you belonged to my verse alone, and my writing was the only place your beauty got its voice. But now another poet has arrived and eclipsed me—my work has gone stale, and you've turned to his words instead. I'll admit he's worthy of the job; you deserve a better pen than mine.
Here's the thing, though: whatever he praises in you, he's really just describing what's already there. He tells you you're virtuous, beautiful, talented—but he plucked those words straight from watching you. He has no praise left to invent. Everything he gives you in his poem already lives in you.
So don't thank him for his verses. He's not truly giving you anything new. You're the source; he's just the messenger. The credit belongs to you, not to him.