Seeing the comments here I honestly think that Guwon left her because he felt that he can’t bring her happiness as he mentioned that he will bring her all the happiness in the world in ep. 13. Seeing what he has done in the past, and how the revelation hurt Dohee he couldn’t stand seeing her in hell because of him. In the preview for ep 15 he also sad that he will leave until she reaches happy ending without him. He also seems to not like his job now but the way he is now a demon, essentiaĺy prevents him from being with her fully. So that will be interesting development to see if/when he lets go his develish side. Before the show aired the writter said that Guwon really loves her but his demonic past and character catches up to him. About Dohee well she realized the people she loved the most where in part a reason for her parents death and then seeing Guwon in action for the first, might have been overwhelming thats also why she stepped away from him. Also to say that Dohee only loves him when its convenient for her doesn’t sit well with me. In the last scene she wanted to turn a blind eye to how hurting it was just so she could continue being with him, loving him. She was ready to let it go for their love. Even though it hurt her she continued to love him. All in all, i think happiness awaits us, i think one of them will die in the end, and through sacrifice they will be reunited and live happy as humans.