Why was persephone not permitted to leave hades permanently?

What theme is the main theme of this excerpt from the metal pig by hans christian andersen? in the garden of the grand duke's palace under the roof of one of the wings were a thousand roses bloom in winter, a little ragged boy had been sitting the whole day long. the boy might serve as a type of italy lovely and smiling and yet suffering. he was hungry and thirsty but no one gave him anything and when it became dark and they were about to close the gardens, the porter turned him out. a long time he stood musing on the bridge which crosses the arno and looking at the glittering stars that were reflected in the water which flowed between him and the wonderful marble bridge trinita. he then walked away towards the metal pig, half note down, clasped it with his arms, and, putting his mouth to the shining snout, drink deep droughts of the fresh water. close-by lay a few salad leaves into chestnuts, which register for his supper. no one was in the street but himself. it belonged only to him. he boldly seated himself on the pig's back, leaned forward so that his curly head could rest on the head of the animal, and, before he was aware, fell asleep. it was midnight. the metal pig raised himself gently, and the boy heard him say quite distinctly, hold tight little boy, for i am going to run, and way he started for most wonderful ride first they arrived at the p as the del granduca and the metal horse which bears the dukes statue made allowed