What is "The Nymph's Reply" all about?
This poem cannot be completely understood without also reading Christopher Marlowe's poem "The Passionate Shepherd to His Love." There is an excellent summary of "The Nymph's Reply" here are eNotes, and I've some of it here:
In the Marlowe poem, the shepherd proposes to his beloved by portraying their ideal future together: a life filled with earthly pleasures in a world of eternal spring. Raleigh’s reply, however, debunks the shepherd’s fanciful vision. While Marlowe’s speaker promises nature’s beauty and a litany of gifts, Raleigh’s nymph responds that such promises could only remain valid “if all the world and love were young.” Thus, she introduces the concepts of time and change. In her world, the seasons cause the shepherd’s “shallow rivers” to “rage,” rocks to “grow cold” and roses to “fade.” The shepherd’s gifts might be desirable, but they too are transient: they “soon break, soon wither” and are “soon forgotten.” In the end, the nymph acknowledges that she would accept the shepherd’s offer “could youth last” and “had joys no date.” Like the shepherd, she longs for such things to be true, but like Raleigh, she is a skeptic, retaining faith only in reason’s power to discount the “folly” of “fancy’s spring.”
See also the eNotes study guide on Marlowe's poem, linked below.