Why could Persephone not return to the land of the living?

After Hades had abducted Persephone, Persephone's mother Demeter (goddess of harvest and agriculture) neglects the earth in her grief. read more

This results in famine all over the world, and Zeus is forced to order Hades to return Persephone to her mother. Before freeing her, however, Hades offers her a pomegranate, which she eats. Because she has tasted a fruit of the underworld, she is obliged to live in the underworld for a third of the year. read more

Zeus got panicky because the world fell barren while Demeter wasn't there, crops didn't grow, etc. so he bargained with Hades to get Persephone back to the living world. Because she'd eaten the food of the dead, she was only allowed to return for a period of time, then return for a period of time. read more