Kyungsoo lives in the moment and Jongin in the past, for love letters are written in the same language as suicide notes, and goodbyes aren't always the kind that are sad; although for Jongin, they almost always are. 

Extract

Jongin finds beauty in the simplest of things; the smell of freshly brewed coffee in the morning. The sound of leaves scattering in the Autumn wind. The feeling you get when your toes first come into contact with that first ripple of cool sea water, and that feeling you get as you watch the sun meet the horizon in a passionate exchange; a haunting and brief kiss that lights the sky an array of oranges and reds and all the colours in between. But most of all, he finds beauty in Kyungsoo's smile. The way it stretches over his lips, they way it forms a sweet, soft heart. The way the corners of his eyes scrunch up like paper, and the way the irises in his eyes almost sparkle-- and mostly, the way he's never gotten the chance to see anyone else smile, yet he still knows how. That, Jongin thinks, is true beauty.