Sir Walter Raleigh wrote this poem as a response to Christopher Marlowe's poem, The Passionate Shepherd to His Love. In Marlowe's poem, the shepherd woos his love by making promises of an idyllic pastoral life and all sorts of material benefits if she chooses to be with him.
In The Nymph's reply to the Shepherd, Raleigh's nymph rejects the shepherd's advances since she finds his promises unsatisfying and hardly commensurate to her expectations of what an ideal relationship is like. Her comments clearly indicate that she finds the shepherd's promises limited since they are restricted to material values and do not relate to true commitment and loyalty at all.
The nymph is critical of the shepherd's honeyed tongue and suggests that it could disguise an acid and cold heart. The words promise much, but are only that. The shepherd hardly mentions what he actually feels and he might be devoid of true passion.
A honey tongue, a heart of gall,
Is fancy's spring, but sorrow's fall,
The nymph rejects each of the shepherd's promises and emphasises the transient nature of all that he mentions. None of the promised delights he mentions are permanent and all are bound to die, fade away or lose their value over time.
What the nymph needs is something permanent, greater than the transitory, something that transcends the periodic nature of things. In her reply she states:
But could youth last and love still breed,
Had joys no date nor age no need,
Then these delights my mind might move
To live with thee and be thy love.
The nymph therefore rejects the shepherd's appeal. She desires things which, one may believe, are impossible to attain: (ever)lasting youth, a regenerative love, permanent, timeless and inconsequential joy. However, although her request may seem paradoxical, her wish is not an unrealistic one. She seeks true happiness throughout her lifetime, i.e. her desire is that these qualities should endure during her stay on earth with the shepherd as her companion, and this is a promise he does not, or is unwilling, to make.