This drama could've been a very nice, lighthearted one, but I don't know what the writers thought. The pace for 1-14 episodes was mind-blowing, as if it kept one interested for further episodes, but the last 2 episodes went downhill.
This drama is about their youth. One wanted to do what she loved and the other had to give up on his dreams to survive. They both met due to an incident, and that the friendship they developed was worth it. Meanwhile, NHD also made friends with her idol, and that bid was THE BEST in friendship. The second lead couple, no triangle, just a sweet, innocent, and non-giving-up relationship. And the last JSW, she is gold. Her personality and her confidence are so cool.
Apart from that, NHD and BYJ slowly and steadily form a love relationship. It was all good until BYJ made it into the local news. From there, the NHD's part of the problems began. And when BYJ had to go to NY, it all changed and it led them to breakup.
Honestly, I was fine with the break up. They had their own opinions and they made sense too in the last episode when they burst open. It's just that something is missing in the end. I don't know how a breakup that is 20 years old still haunts a happily married woman. Of course it was her youth, but still, the ending wasn't that satisfactory.

On the other hand, the title "25–21" was just about their youth. The past that she lived and which has nothing to do with her present. If we think that way, the gap for her present life for us can be filled.