How does the Nymph's Reply follow up Marlowe's original proposal? What assumption was assumed in the beginning of the poem?

The Nymph also mentions that IF love would last forever, and also all the things that the Shepherd offers her--the belts and kirtles and beds of roses, etc.--then she WOULD accept.  This is true to the carpe diem attitude of the time period, and also to the category of pastoral poetry, to which these poems belong.  We know, however, as the Nymph does, that these things do not last forever, so her very realistic answer is "no, I will not come with you and be your love."  Raleigh follows Marlow's poem stanza by stanza and matches it in tone, subject, and form/structure.