One day, Haides, God of the Underworld, saw and fell in love with Her, desiring Her to be His wife. He went to Zeus and asked that He be granted this honor, His daughter's hand in marriage. The King of the Gods agreed and Haides too Her down into His confines, to rule the Underworld as His Queen, all of this having been done without the consultation of Demeter, as a mother's consultation was not required at this time in history.
But upon hearing this, Demeter was both devastated and furious. For 9 days and nights, She searched over the world, but without success. Then Helios, the Titan Sun God, loyal to Olympos, told Her what had happened, as He was able to see everything. Demeter knew She could not overrule Zeus, but She could use Her own personal leverage. And thus She declared, until my daughter returns, nothing shall grow upon the Earth. I shall remove my presence and all shall die.
Zeus then called Haides and Demeter into negotiation, and it was agreed that for half the year, Demeter would be reunited with Her daughter, while Persephone would spend the other half of the year with Haides below. During the latter, winter and death covers the Earth. But when Persephone returns, so does Demeter's presence, and spring and summer reign. Thus, Persephone became, among other things, the Goddess of spring, and Demeter the Goddess of the seasons.