She gave her heart away too many times and was given nothing in return.  The only thing left for her was hopelessness and the promise that no one would ever love her in the way she wanted to be loved.  She eventually gave up on the idea and closed herself away.  She had no desire to let her heart be broken again.  Yet when he comes along, she can't help but let herself confide in him.  He understands the pain of a broken heart.

He was the only one that knew what it felt like to not want to feel.  When she had nowhere to go, no one to turn to, he was there even when she didn't want him to be.  So he helped her in his own way, even if it would kill her in the end, but she didn't care as long as she died the way she wanted to.  To Kang Seulgi, death by Kim Jongin didn't sound too bad.