a sad ending that warms the heart ?

Don't watch it if you can't stand sad endings. I just watched it for that and honestly, I'm satisfied. I really liked the actors and the story. I especially liked Kim Yoo-jung's performance, as the fantastic Kim Bo-ra, and Baek Woo seok, the guy I found the most mysterious and intriguing in the movie. But I didn't really understand why and how he died. Maybe I missed something important or I don't know. In any case, the ending really left a mark on me and I will remember it for a very long time ?