Hi, just jumping in here to share my theory for fun!
Yi jin and Hee Do realized that it is love at 25 and 21. It was a healthy relationship built on mutual respect ❤️, however, the "times" decided to get it away from them. It could be a problem from Hee Do or Yi Jin's family.
They believe that it's best to support each other from a far. Just like how Hee Do did not get angry at Baek Yi Jin suddenly leaving when they're 22 and 18, and instead, still rooted for him and prayed for his happiness!
So, both of them focused on rebuilding their lives.
I think this separation could be from Hee Do's side, as they first separated at 22 and 18 due to Yi Jin's side.
So, fast forward, 5 years later. All seems well for the both of them and it's destiny that they met again.
At 30 and 26, both are adults and still love each other. However, it seems like "times" have changed and it changed them as well. There were also unresolved ghosts from Yi Jin's family and past.
Or, there's a dream that is worth pursuing. So, they separated for good—tying all loose ends. They were happy for that short time, and once again, respected each other and chose to move forward. In episode 4, I do not think Hee Do resents Yi Jin or anything in the present.
Hee do learned that she's pregnant, but she chose not to tell Yi Jin due to the "times". The "times" could've been "bad" or an "opportunity" to Yi Jin, that's why she did not disclose he's Min Chae's father. She believes that it's the most appropriate on the situation.
Her mother also knows that as well and that could been the reason why she asked Min Chae in episode 4 if she knew Baek Yi Jin or the person in the photo. She did not agree at her daughter decision at first but respected it.
In the present, Yi Jin is also alive and doing well just like Hee Do. In this theory, it would be so dramatic to make Yi Jin have his own family. 😆Soooo, he still doesn't have a family and diligently works hard at his current career!
As Min Chae reads her mother's diary, she learned that her mother only loved one person and that is Yi Jin and starts to put things together. Hee Do and Yi Jin will meet again and will reminisce their past.
The daughter learns that Yi Jin is her father, but she's not angry.
I think this is really possible because we don't really see the father. Min Chae will ask Hee Do about this and would like to meet Yi Jin.
Hee Do and Yi Jin will reconcile and rebuild their friendship in the present times and the ending will be an open to interpretations if they will get back together.
Of course, even though it's open ending, Yi Jin will be happy knowing that he has a daughter and wants to be a good father. Seeing how he's family oriented, there's a higher chance they will now have a good family as the "times" opened that to them! 🤗
So, that's it!
Note: I got this theory from everyone else who believe (1) Yi Jin is Min Chae's father but he doesn't know and (2) Min Chae will make Yi Jin and Hee Do get back together. I just waved it all together to arrive at this conclusion.
I think the "times" will also play a very important role in their love story, as it did in their dreams! 🤗 I think the "times" playing on people and our relationships with our loved ones is harder to conquer than on how it plays on our dreams.