"The Nymph's Reply to the Shepherd" is the nymph's reply to the shepherd's request for her to come be his love. Unfortunately for the shepherd, the nymph is simply not "feeling it"!
The nymph, in this poem, informs the shepherd that she cannot love him and she gives him her reasons for this. Among those is that love is not eternal. She alludes to the fact that love is too fleeting and that he cannot guarantee he will love her forever. She appears to have trust issues, as many of us can relate to that have been hurt in relationships. She longs for a perfect and ideal love; however, she is realistic and does not want to take the risk of loving him and being hurt.
The enemy in the poem really isn't an enemy at all; however, it prevents her from being able to go into a relationship with the shepherd. Distrust prevents the nymph from pursuing this relationship. It is also important to note, though, that traditionally in literature, relationships between mortals and nymphs were tumultuous at best!