I started this drama when it first aired and I wasn't really impressed. Neither by the scenario nor by the actors so after a couple of episodes I just dropped it out. I recently started it again and I have to say it got much better somewhere on the 8th - 9th episode because the plot started changing dramatically. I don't want to make spoilers BUT the final 3 episodes have no connection to the rest of the drama. For me they were the best part. I cried A LOT and so you will you for sure !

So the drama is in 3 phases : high school , mid 20s , early 30s. Although the first phase is very boring and so is the second , I think it's all because of how immature the main characters still are. Once they reach their 30s and their lives have completely changed the story becomes deeper because they will have to deal with serious problems and face their demons like proper adults. If I would re-watch it , it would be only for the final episodes.