“My verse alone had all thy gentle grace; But now my gracious numbers are decay'd”
'Numbers' is a Renaissance term for verses — lines of poetry counted out in metrical feet. 'Gracious numbers' means pleasing, harmonious verse. The phrase says the speaker's poems once had all the beauty ('gentle grace') they needed when he was the youth's only poet, but since the rival arrived, that quality has drained away.
historical The sonnet form“Deserves the travail of a worthier pen”
'Travail' means hard labor or toil — it is a more intense word than 'work,' carrying a sense of painful effort. It is distinct from 'travel' (a journey) in modern English, though in the 16th century the two spellings were interchangeable. The speaker is conceding that the youth deserves a poet who will labor seriously for him.
- TLN 1105rhetorical device
“Then thank him not for that which he doth say, Since what he owes thee, thou thyself dost pay.”
The closing couplet delivers the logical conclusion of the paradox: the youth owes the rival poet no gratitude, because the youth has effectively paid for every compliment himself — the raw material (his beauty, his virtue) was always his own. 'What he owes thee' means the debt the rival incurred by borrowing those qualities; the youth 'pays' it by being what he already is.
historical The sonnet form