To be specific, the ending is fine with me.
See I wouldn't mind it as much if I didn't like the main story so much. So to see THAT, and then be told that's their past life on TOP of it??? Makes what I thought a totally adorable story something beyond recognizable
Welllll
It's their past lives, I doubt she remembers the atrocities she did during that time as it's not specifically stated that she planned and manipulated everything in the main story because she remembers. In a sense, the implication of the ending is that, they are always fated to meet each other in whatever lives they're in. It's just that, Sia's soul strives to choose an identity that distances herself from Heeyoung (as to not hurt him).
Nah I won't lie, I'm fine with the side stories.