Demeter and her daughter Persephone were inseparable and loved each other so much one couldn't bare to be to be apart from the other, but Hades was in love with Persephone and wanted to have her for himself.
When Persephone wandered off from her mother's protective gaze one day, Hades discovered the perfect opportunity to get Demeter's lovely daughter for himself. He opened up a crack from the Underworld to the world above and kidnapped Persephone and took her to his home in his chariot.
Demeter grieved for her lost daughter and refused to let any nature stay alive until nature until Persephone was brought to her safely.
Persephone, like Demeter, missed her other half so much while still living in Hades's house and hated being trapped in the Underworld far away from her mother and any source of nature besides the pomegranate tree of the dead. Hades however, loved having Persephone as his queen in the Underworld and was happier than he had ever been.
Demeter came across a boy who claimed to have heard a girl screaming and there being a crack in the ground, so Persephone sent Hermes to the Underworld to retrieve her daughter. Sadly, Persephone in her despair had eaten the food of the dead and would have to return to Hades.
Demeter didn't want to give up her daughter to Hades again so the pair went to Zeus for help on the matter. He came up with a plan for Persephone to go to Hades every year for a month per each pomegranate seed she ate. This time each year is our winter while Demeter grieves for her daughter.