"The Passionate Shepherd to His Love" takes place where the grass is always green, nothing ever dies, and nature is in complete harmony with all of man's whims, needs, and desires. So it's no wonder that the speaker uses nature and all its awesomeness as a convenient way to woo his lady love. Is there anything sexier than… sheep? Okay, so there are plenty of things sexier than sheep, but for our speaker, the pastoral world might as well be the most romantic restaurant in town.
Marlowe presents an idealized picture of nature in an attempt to satirize the unrealistic visions of the countryside held by city-dwellers. Basically, he's making fun of the folks who think the key to happiness lies in roughing it.
Marlowe's choice to portray a pastoral world signifies his dissatisfaction with modern society and urbanization. It is a veiled longing for a return to simpler times.