In "The Nymph's Reply to the Shepherd," the nymph dismisses the shepherd's idyllic vision of their life together by shining a light on the facts of life.
The nymph must doubt the shepherd's promises, as one of her conditions for being the shepherd's lover is if there was "truth in every shepherd’s tongue." In "The Passionate Shepherd to His Love," the shepherd promised lovely gifts that a shepherd would not typically be able to afford, such as a belt with coral and amber in it. Therefore, one of the main flaws the nymph points out is that the shepherd is obviously lying to her.
Another fault in the shepherd's logic is that he promises good tidings and happiness that would only be available to them in summer and spring.
And we will sit upon the Rocks,
Seeing the Shepherds feed their flocks,
By shallow Rivers to whose falls
Melodious birds sing Madrigals.
In the above passage from "The Passionate Shepherd to His Love," the shepherd says they will sit out in the open and enjoy all the natural world around them, but that would only be possible in the warmer months. No one would want to sit on a freezing cold rock in the dead of winter because they could become sick or die from the cold.
Moreover, none of the shepherd's promises speak of the hard life a shepherd must lead to sustain himself and his family. He talks of watching other shepherds and their flocks of sheep but what of him? He should be tending to his sheep, too, instead of sitting on a rock with his love, right?
Thus, it comes as no surprise when the nymph states the main conditions needed for them to live together are eternal youth and summertime. If such things were possible, she still gives the shepherd no promises. If they had youth and summer forever, only then those "delights my [the nymph's] mind might move." She only says her mind might change because she does not trust the shepherd, which is in line with the pragmatism she displays throughout this poem.