Compare the structure of "The Passionate Shepherd to His Love" and "Nymph's Reply"?
The passonate shepherd presents the ideal of Romantic love. The images are soft and mostly from nature (myrtle, flowers, roses, poises etc). The Nymph's reply presents a more real side of love. It makes the case that this would be nice IF the events and objects in the poem would last forever ... but they do not.
But could youth last and love still breed,
Had joys no date nor age no need
But the fact is that youth nor any of the joys mentioned in the shepherd's poem will last. If love is to be "real," it must be based on things that will last, not romantic ideals, although these must have their place in a relationship.
An interesting presentation of the idea of love as lasting is in Shakespeare's sonnet "Let me not to the marriage of true minds admit impediments." It might be interesting for your comparison.