Other people have expressed very eloquently the utter heartfelt perfection of this drama. You have not one but THREE sets of utterly perfect chemistry. You have a gorgeous, killer OST, You have the most charming moments of beautiful poetry balanced by some genuinely funny moments, all of which, rather than via some contrivance, come from the characters and your investment in them. At times this drama has so much hurt it feels difficult and you wonder "can I get through this pain"? If you ever want a drama with which to introduce friends to K-dramas, this is it. It goes to such emotional depths that to label it a "rom-com" is entirely laughable.
And... did I mention it has the best Ailee OST since her wondrous inclusion on the Goblin OST?