This is Hades. He is the god of the underworld. Although he is very rich, he is unhappy, and lonely.
I am so sad and lonely here, in the dark Underworld. I'll go take my three headed dog for a walk.
As Hades was walking Cerberus, He came across a beautiful young woman.
Oooo la la! I shall take her with me to be my wife!
What a cute dog!
Hades took Persephone back to the underworld with him. His plan was that he would keep her here until she ate, and then she would be his wife forever.
At last, I wont be lonely!
Hades locked Persephone in a room, and offered her all kinds of delicious food, in hopes that she would eat something, and stay there forever. Desperate from hunger, Persephone ate six pomegranate seeds.
What Hades wasn't counting on, was Persephone's mother, who was the god of the harvest. When Demeter was happy, the crops would flourish and bloom. When she was angry, however, the crops would wither and die. She demanded her daughter back.
I WANT MY DAUGHTER BACK!
Together they worked out a deal that Demeter would get Persephone in the spring, and Hades would get her in the winter. When Persephone was with Hades, in the Winter, Demeter was sad, and the crops would die. When she was happy, in the spring, the crops would flourish. That is why there were seasons, according to ancient Greece.