I can't tell you exactly why, but I really liked this movie. It's pretty complex, it's story lines interfere with one another, there are many things left in the open (or so I think) and the only thing I can say for sure, is that I didn't understand everything shown in the movie.
It builds up from the beginning and many things get revealed much later (like the relationships between persons, story lines etc.). The performances are great and the characters are all fully explored (even if their intentions/motives might seem weird). (South) Korean movies in general are sometimes a bit weird or seem so, when watched by western audiences, but this one is weird for anyone ... and I just like it for being as it is.