We've said it before, and we'll say it again: "The Passionate Shepherd to His Love" is a poem set in the countryside, and not just any old countryside at that. This poem is a pastoral poem, which means the version of the countryside it depicts is a little bit too good to be true in real life.
"The Passionate Shepherd" also focuses in on the countryside in springtime because, if you're already planning on eliminating all the unpleasant aspects of living in the country, why not just go ahead and present it in its best and prettiest time of year, too. Never mind that winter will come and kill off all those flowers and the baby lambs that are so cute now will eventually be turned into some sort of stew.
But the springtime setting serves a double purpose; all the references to budding flowers, baby animals, and the month of May not only set the scene, they also emphasize the new life and fertility associated with the springtime, which is not a bad thing to mention if you're trying to convince someone it's a good idea to take you as a lover and to move in with you.