You will find a very good outline of the poem here. As this outline says, Marlowe was writing in a very old tradition dating back to classical times in which shepherds and other rural people address someone who might or might not be physically present. In this case there is no mention of the 'love' being actually present. The shepherd makes very extravagant promises to her in his appeal and the impact would be very much reduced if this was narrated as a story rather than coming from the shepherd himself. Depending on how you read it, he can appear a bit desperate, perhaps in a sexual manner, and even rather ridiculous, and it is quite possible that this was Marlowe's intention. Indeed it is quite possible that the youthful Marlowe was sending up the whole bucolic poetry genre in this poem.