Walter Raleigh wrote the poem "The Nymph's Reply to the Shepherd" in 1596, in response to the 1593 poem written by Christopher Marlowe entitled "The Passionate Shepherd to his Love."
In order to understand the themes of this poem, it's necessary to have a background in the history and customs of the time. Elizabeth I was on the throne of England during the time both poems were written. She revived the medieval custom of courtly love. Courtly love involved a man pouring out his passionate and unending love for a woman, who would respond with rebuffs and often disgust. It became a game of cat and mouse and created a tension in the relationship that was culturally acceptable.
It was extremely important during this time in history for a woman to maintain her purity until marriage. For this reason, no respectable woman could have seriously entertained the shepherd's proposal in Christopher Marlowe's poem, since he promises nothing of marriage and its requisite dignity, only love and passion.
Raleigh's response to Marlowe's poem takes the perspective of a woman who is following the rules of courtly love. She is also noting the fleeting manner of young love. All the things that the shepherd is promising only last for a season and then are gone. She implies that the shepherd is not truthful in what he promises or that he doesn't truly understand what it is that he is promising. Therefore, one of the themes is that love (or passion) without truth is not worth pursuing. Consider the nymph's reply in the first two stanzas of the poem:
"If all the world and love were young, and truth in every shepherd's tongue, these pretty pleasures might me move to live with thee and be thy love.
But time drives flocks from field to fold, when rivers rage and rocks grow cold, and Philomel becometh dumb; the rest complain of cares to come."
When the nymph says in the first stanza that if love stayed young, maybe she would be moved by the shepherd's words. But youthful love doesn't last any longer than the seasons, so she will not accept his proposal. In the second stanza, she talks about time driving flocks from the field to the fold when rivers rage, like they do in springtime with the melting of the snow. And rocks grow cold in winter time, another reference to changing seasons. Philomela is alluded to in this stanza. Philomela was a character from Greek mythology who, as recorded by Ovid, was the sister of King Tereus. The King's wife wants her sister to visit, and Philomela's father is reluctant to let her leave. Philomela's father asks Tereus to accompany her and protect her like she was his daughter. Tereus agrees, but is inflamed with lust and rapes her. Then he threatens her, and when she is defiant, cuts out her tongue so she can't expose him. The nymph's reference to this myth shows a shrewd skepticism of the promises of lovers driven by lust.
Another theme of this poem is the decay of youthful passions. Consider stanzas three and four:
"The flowers do fade, and wanton fields to wayward Winter reckoning yields; A honey tongue, a heart of gall, is fancy's spring, but sorrow's fall.
Thy gowns, the shoes, thy beds of roses, thy cap, thy kirtle, and thy posies, Soon break, soon wither, soon forgotten, In folly ripe, in reason rotten."
The nymph points out that all the things the shepherd has promised to give his love soon fade away. A tongue that pours out honey today later becomes a heart that is bitter as gall. The gowns and shoes made of natural things will wither and die. They're full of folly, or foolishness, and if one subjects them to a reasoning mind rather than a foolish heart, they turn out to be rotten, losing all their beauty and appeal.