It's always interesting to see high concepts play out for romantic dramas, immediately kicking into action with high stakes, date or die. For a fantasy to be immersive without too much distraction, there needs to be in world explanation for important story elements.

Setting up the rules for kissing the birthmark that was the wound site from a previous life to calm some sort of pain that activates is interesting. It is unclear if it's their proximity only that sets off the pain or if Dong Baek always had that pain and only Yudam can cure it. And so much more like a there needs a clearer explanation exactly what went wrong and got Yudam and Dongbaek cursed, was Yudam alive for 300 years consequtively or was he reborn with the same memories, who's the spirit that possessed Dong Baek, did Dong Baek have other life times in between the first and the current, is the heart shaped mark from a different stabbing from one of those other lives or else how did Yudam know about it, is Denis the assassin that killed Dongbaek, why is Denis calmed by Dongbaek, why is he obsessed with him...etc. It's a really fascinating premise, but a lot felt like it was left on the cutting room floor. I like how the past life scenes show Yudam as warm and kind, juxtaposing with his self 300 years later after he has been living on autopilot, how he's been thawing with his time with Dongbaek, and finally he's back to how he was in his first life after he forgets Dongbaek. It's a bittersweet ending that Dongbaek gets to remember first by keeping his memory in this second loop, but Yudam is already interestested so they can start over with Dongbaek explaining what happened to him eventually. They're even that way with Dongbaek not remember their first life either, neither will have the trauma from that life, though Dongbaek will have some from dying in the first loop of the current one.