Might Contains Spoiler

I liked the end as it give us freedom to make our own conclusions. 

Some people might think it's rushed as we want to dive more in this story world.

I don’t think it was too rushed, but I think they could have cleared up some massive questions. Some of the hints didn’t add up: what was NHD and her mum doing in hospital together? Why didn’t they show NHD’s husband - were they happy together? I hope NHD found her happiness. I think NHD and BYJ loved each other purely, but it was “the times” that changed them, and they weren’t meant to be.

I didn’t really understand the tunnel scene either. But in the end, this ending has really shattered my heart. 

Na Hee Do and her mom were in the hospital together because they went in for colonoscopies at the same time. 

They didn't show Na Hee Do's husband because the story isn't about that.

I just finished the show, but I think the tunnel scene was just to show what they each said to each other in those last notes. The words that they really wanted to say (instead of the nastiness that they spewed at each other earlier). Na Hee Do wrote it in her diary, but then lost it. Baek Yi Jin read what she wrote, and then added his piece to her diary, but then it didn't get back to her for YEARS.