A few things makes this my favorite drama:

1) Humor. It's the kind of humor I like and the music fits perfectly.
2) Actors. Well casted and each person plays their part perfectly.
3) No annoying characters. One of the reasons why I might stop watching a drama midway is secondary characters being annoying. If the FL's best friend is too whiny, if a mean girl is too annoyingly mean for too long, or if an ex is too clingy. That doesn't happen in this drama.
4) ML's lines. His sarcasm is absolutely hilarious.
5) Chemistry. It's not just the ML and FL's chemistry, it's the entire cast's chemistry. You can tell they're just having a great time filming this drama. The set must have been a super fun place.

All in all, I've been watching this drama on repeat for like, the seventh or eighth time. It's my new background show, beating out Pride and Prejudice (6 hour version) and Lord of the Rings (extended version).

This and Reset are my only 10.0 rated dramas.