Best Korean drama, hands down. It is very much a highly sensitive, raw, honest drama about vulnerability in human relationships. It's about humanity. And it's crafted to such a high quality and I just haven't seen the two together in a kdrama before.
The characters are written with such sensitivity and care. They are complex and are neither good nor bad, just as in real life nothing and no one is good or bad. IU's character is such a memorable character. She is strong in a natural and realistic way but also soft at the same time, in the most natural way. Park Dong Hoon is also unforgettable. There aren't many characters I can forget from this drama, and it's been a year or so since I've watched it. Their relationship is something I hope I will one day experience. It is so so precious. It is what humanity is about. No bullshit, just understanding, empathy, honesty, and most importantly vulnerability. Such a beautiful relationship.
I honestly could go on but I'm deciding not to, because I'm scared that in describing it I will oversimplify its weight and complexity.
I find it incredibly hard to get back into kdrama now because I can't help but compare anything else I watch to this, and I know that's silly because not everything has the same genre and themes but I think the show has changed me and my relationship with and taste in kdrama so much that I find it hard to watch the kind of (sorry-- just imo,) shit that comes out these days. It kind of makes me sad because I've been watching since a young age and used to be able to enjoy the superficial stuff but this drama (and a couple other good ones mind you) has honestly changed the kdrama scene for me, and probably for others.
"It's difficult to be around a person you saw so vulnerable."
"Promise that you'll pretend you don't know if you find out about me . . . I'm scared.. I feel like you'll know without telling."
"It's easy for people who live well to be a good person."

[I see some commenting that the show is "too heavy" which, yes, it is definitely heavy, but what's interesting is that it does have this brutal honesty that I understand most will shy away from. Because entertainment predominantly exists for escapism and forgetting life's heaviness. Interesting though, because the sheer amount of those comments really says something about fiction vs reality, and how we turn to entertainment/fiction to seek "something else" in attempt to escape the mundanity and pain of life. (Margaret Atwood refers to this as wanting "something to happen" more than the "breakfast" of life in her speech, 'Spotty Handed Villainesses') Which is what this drama is shedding a light on. In my opinion, the best pieces of art hurts, confronts, shocks, shakes, and questions you and your beliefs.
“I think we ought to read only the kind of books that wound or stab us. If the book we're reading doesn't wake us up with a blow to the head, what are we reading for? So that it will make us happy, as you write? Good Lord, we would be happy precisely if we had no books, and the kind of books that make us happy are the kind we could write ourselves if we had to." (Franz Kafka)]