Symbolism, Imagery, Allegory
Me Before You is a total romance novel. How could Paris not be an important locale?
For Will, Paris represents the life he's no longer able to enjoy. He wistfully describes the beauty of drinking coffee and eating croissants in quaint Paris cafés, but recoils when Lou suggests that they go there together. He believes that visiting this magical city is his current state will only sully his memories of it.
So it's only fitting that Will demands Lou visit the city after his death. How does he get her to go? Well, he writes her a letter and then requests that she not read its contents until she goes to a specific café and orders a specific meal—croissants and coffee, obviously. In this way, Will is trying to imbue Lou with the same sense of exploration and adventure that he enjoyed so much before his accident.
Does it work? Well, check out the closing bit of that scene: "And stepping out from behind the table, I straightened my bag on my shoulder and set off down the street toward the parfumerie and the whole of Paris and beyond" (e.32).
Yeah, it worked like a charm.
Will knows that Lou will be devastated by his death for a long time, but he hopes that she will take away from the experience a renewed appreciation for the wonders that the world has to offer.