2 men who love the same woman. And now all three of them are there at the same time Seems Doya is the only one who survived hundreds of years while So Won and Jaehyun was reincarnated.
My theory is that Jaehyun tried to kill Doya, but So Won protected him and died instead, which led Doya to make a deal with a demon, or someone made a deal with Doya to make a wish for him so that So Won could be reincarnated anyway since demon mythology shows that demons are bad and nothing good comes from making a deal with them and that you will eventually pay a price for everything the demon gives you. The price could have been Jaehyun also being reincarnated to ruin Doya and So Won's relationship like the past.