The story telling in this drama… Wow! Just wow! I believe nobody can ever replicate such a phenomenon, even The Genius, Kang Full, himself. This show is art. The way it unfolds, OMG.I am not one to cry so easily. The only time I cried watching a Kdrama was when Reply 1988's ending took place, that was years ago. This drama made me cry so much, more than 1988 could ever again. Moving was a masterpiece, but this? This is a jewel more valueable than any jewel has ever been.
huh?! I thought that was it. what could they possibly explore in S3?
Maybe if it was really a dream, then why did he see all of the people present in the hospital, and why all of them went there in the first place. Why did people who died in the dream die in real life too, etc.
The light shop is not present in the real world. He is not the guardian of human beings, as his shop is not in…
Somebody here said he died of hypothermia. Remember, when the accident happened and all of them were pulled to the hospital, there was another guy who was complaining about his health, the doctors told him to go to another doctor. It's him.
They did not technically turn into monsters. They were given 3 days to still be on earth before going to afterlife.
Those are not monsters. Those are spirits of those who died. Monsters have evil intentions and haunt the living. Those who died and those who are on the brink of death are in an alternate reality right now which is the alley that we see.
2 
Some would argue it was the tears in my eyes.
It ended up being the thing from where it started. The writer had a vision, and that is appreciated.