Demeter loves her daughter, Persephone, dearly, and her land will forever be green as long as her lovely, Persephone is always alongside her.
Hades decided it was time to go up to the regular world and see what's going on. He brought his three headed dog just in case.
Persephone saw the three headed dog and started playing with it and laughing. Hades heard her beautiful laugh and fell in love with her. He decided he would take her back to the underworld.
Since Persephone wasn't with Demeter the everything died, and became awfully cold. Demeter missed her daughter and would do anything to get back her back. Persephone felt the same way, for she was locked up, and refused to eat, because of a myth she heard.
Zeus sent his son to make a great deal with Hades, so that Demeter could get her daughter back, and all the crops would grow back and be green again.
Zeus's son made a deal where Persephone would see her mother for six months. While she was with her mother everything was green. But, the other six months she was Hades and everything grew sad and died.