In one (more romantic) version of the lore, the Mystic Realm awarded Hou Yi (the husband) the Immortal Pill/Potion after he prevented the earth from being scorched. He and Chang'e were going to both take the potion so they can be together forever. A group of bandits heard about the potion, and broke into their home while Hou Yi was away, Chang'e had no choice but to swallow the potion entirely and that gave her divine powers in addition to immortality, she was able to escape and ascend into Heaven, and because she's no longer human, she and Hou Yi were forever separated...