Oh gosh... I have a bunch of explaining to do... But you probably won't understand it if you haven't watched the show.. And it'll be a pain to type on this DS, so I'll do a poor attempt at paraphrasing. Basically, the boy had donated his heart before he died. The girl's reason for staying in the 'pre-afterlife' was so that she could thank the person who saved her life while she was still living. Once she found him, she had fulfilled her purpose for staying in the 'pre-afterlife'. So she had no choice but to leave once she had found her peace.