Story: Saying that the storyline is awesome is underrated. There is comedy, there is melodrama, there is thriller, pretty much the seasoning we need for a great drama. Then of course the unexpected twists in the plot to keep us on our toes (not literally).
Great cast? Hyun Bin and Son Ye Jin. So, yes. The rest of the casts are great too. But we always care for the leads, don't we?
Music? K-drama always have great music, so of course.
Rewatch? Of course, who can get over this drama? Even not rewatching this, my brain is on autoplay. Random scenes would pop up when I'm driving, showering, eating. Pretty much like how you'd think of your ex when you break up. This is real.

Wait, still not sure to watch this? Maybe you could try Goblin. That's another drama I had a hard time getting over, maybe I'm still not over it either. That one has people with superpower involve, so if you're more into that, then go ahead. Once you feel like watching mere mortals then you can come back to this drama.