This is about an old chinese romantic story. the leading roles are Ms. Ju and Mr. Lai. They loved each other, but their parents disagreed with this development, and destroyed it. When they became aware of the truth that they couldn't fall in love, both of them were going to die by committing suicide.
After dying, they became a couple of butterflies. They were invisible. Nobody could see either of them, even themselves. When they were alive, they had promised that if they could die together, they could meet in the nether world. Therefore, that promise didn't come true.
So, they couldn't be close continuously, not only when they were alive, but also when they were dead. We say that it's a miserable story. Misery can be remembered longer.