This show gives a warning about suicide at the beginning but it happens in ep 11, and ep 12 is about what happened if Min-ju never committed suicide.
The difference between sad and happy ending is shown to make us all think about how we treat people in our lives.
One person's act affected so many lives around her, and yet she though of herself to be so insignificant ....
She wasn't a bad person without any hope for redemption. No. She was just an emotional teenager that nobody understood and cared about.
The time travel, the search for the love lost in time ....all of that wouldn't have happened if someone was there for Kwon Min-ju to help her overcome a difficult time in her life...
This story is teaching us that every single life and person is precious, no matter how strange that person seems. We should care about the weak and shy and withdrawn persons not because we pity on them, but because they are human, just like us and try to accept, understand and include them.
Also, it is teaching persons who are like that to not overthink, not to try too hard to be accepted and to seek for help and support in people around them. Min-ju just couldn't see that people like that are already around her.
The story is showing us that Si-heon became a friend of In-gyu. Without that friendship he would have probably become to be the same as Min-ju.
Anyway...this drama is great and I hope it leaves everyone thinking about persons they meet in their lifetime as well as themselves.