Twenty-Five Twenty-One is going to be a drama I'll rewatch whenever I get the chance to (after Happiness) because I don't think you understand, I'm OBSESSED.
To be honest a lot of viewers are very unhappy with the ending and I empathize, but as a very INFP person (lol) this was one of the most realest, pure ending of a relationship, or ending a certain chapter in your life. It was a beautiful poetic line to a romance novel.
The leads fated meeting, fated love, fated breakup, it later was so apparent how much both of the leads have grown and matured emotionally and developed sense of perceptives towards life. Some harsh decisions do destroy the people around you but it's never intentional and these concepts are what I found so endearing and beautiful.

In conclusion, I'll forever live this fantasy of being in the squad of Moon Jiwoong, Ko Yurim, Ji Seungwan, Na Heedo and Baek Yijin. I'll never get over them emotionally. :")