The story of Cancer has something to do with Hercules, the great hero.

Due to the overabundance love of Zeus resulting in Hera’s anxiety and envy, she cursed Hercules, who was the son of Zeus and a common mortal, to be mad and to kill his wife and did many crimes. Therefore, for compensating the crimes that he had done, Hercules promised to finish the twelve difficult mission asked by Hera. One of them was to kill the big snake in the Amymone spring. During the fighting, Hera sent a big crab to help the snake. Hercules was painful for the clips of the crab. Yet he finally stepped on the crab and kill it with his native great power. For appreciating the help of the crab, Hera made the crab to be one of the constellations in the sky.

Some theologists thought that in the sky, the big crab was just a messenger. The real leading role is Hera. So her anxiety, envy, and the revenge to the son of the mistress influenced the emotional reactions of Cancer that were easy to feel anxious and envious.