A beautiful story...but...and there's a big BUT here...the time loop concept is flawed. Once Jae Woo goes back in time to 2017 and returns again to 2022, from that point onward he will move forward with his life and live it to its fullest...just as he promised to do at the end of the final episode. He is 25 years old in 2022. In 2037 (15 years later), a 40 year old Jae Woo will not be returning to 2022 as a 25 year old in order to travel back to 2017 to revisit Ji Hoon.

In order for the cycle of traveling back in time to keep repeating itself, then alternate timelines must exist...like the parallel universes as postulated by the string theory. Evidently, Ji Hoon's must have believed in alternate timelines. He believed that he will continually meet Jae Woo and spend 3 wonderful weeks with the 25 year old Jae Woo over and over again. Ji Hoon is willing to sacrifice his own life and die to protect the child Jae Woo...and by doing so, he believes that his self sacrifice will continue a cycle where Jae Woo will visit him for 3 weeks every 15 years. For this to occur, alternate timelines must exist. Without ascribing to the the concept of parallel universes, the time loop concept of this series is seriously flawed.

Despite the questionable time loop ending, it's a beautiful series...and I highly recommend it.