"Summer Love" that's what came to my mind while watching the relationship between Doo-na and Won Joon. They were from different worlds with different destinations who crossed each other's path momentarily by coincidence and created a lifetime of memories.
The drama's ending is sad by itself. But there's a real ending hidden in plain sight for the viewers to find. That's what made me give this drama a 10/10.

*Spoiler ahead*
It's mostly me nerding out about the ending ahead, so tread carefully if you still haven't watched the drama.
In the end we see Won Joon and Doo-na seemingly reconciling with each other but right after we see that they are going their separate ways, each to their own goals. From this it seems like that the final hug was a closure. But there are small clips at the beginning of each episode, right after the opening song where we can find the actual ending of the drama. The most important ones are in episode 1, 2, 6,7,8 and 9. It's a scene of both of them standing across each other at a rail crossing in japan (which can be confirmed by the Japanese letters in the signboards). In the clip of episode 9 the two of them can be seen standing in field of flowers with the mount Fuji behind them and they are slightly smiling at each other. In episode 2, 6 and 7 Won Joon is wearing the same outfit. In the clip in episode 8 and 9 he is wearing the same outfit he was wearing at the end of the drama. From this we can see that their meeting in Japan wasn't actually once but at least twice. But the final clue is actually in the video of the opening song where we can see Doo-na walking on a beach with Won Joon, wearing the same outfit she was wearing while standing in the rail crossing. And at the end of the video we can see them standing on the beach looking at the sunset wearing, you guessed it, the same outfit. From all this it leads us to a conclusion. They are still dating secretly. The drama ending makes us believe that they broke up. This represents them deceiving the people around them including us into believing that was the end. But the hidden clues all across the drama shows us their real situation. Because they can't openly date in Korea because of Doo-na's fame they continued their romance in japan, far from the prying eyes. Also the last big piece of clue is actually also in the opening music video. The whole friend group along with Won Joon's brother Kook Su jin playing around in the same beach. Which makes it clear that they are finally hanging out together again and Doo-na got the friends she always desired and she's not lonely anymore.