One of my favorite myths of all time is that of Persephone and Demeter. It's the myth about how the seasons came to be.
Young Persephone (daughter of Demeter and Zeus) was picking flowers with her friends. She wandered off and suddenly the ground beneath her opened and Hades abducted her into the Underworld to be his Queen. Demeter was so grief-stricken that she wandered the Earth in search of her daughter and ignored her principle duties of replenishing the Earth (she's the goddess of harvest)...and everything began to die. Zeus was so troubled by this he demanded Hades return Persephone to her mother, but before Persephone left, Hades offered her a pomegranate seed, which she ate. Because she ate a food of the Underworld she could never fully leave...so Zeus struck a deal...Persephone would stay with Hades for half the year and Demeter could mourn during this time (fall and winter); the other half of the year Persephone would return to her mother and Demeter would have to replenish the earth (spring and summer).
When my family first moved to California and I came to visit I felt like I was traveling between two worlds: one frozen in winter and another suspended in eternal sunshine. Now that I'm in San Diego "full-time" I imagine this is what it would be like if Persephone had left the Underworld without any hiccups. It would be beautiful all the time but we would miss out on the wonderment of the seasons. Winter is a time of reflection, rest and replenishing; without it we move forward with no break in the routine and we risk burning out. There is some discrepancy with the myth as to whether or not Persephone ate the pomegranate seed on purpose...maybe she liked the idea of being Queen (who wouldn't?); maybe she wanted to prove that she was grown up and ready to live on her own, or maybe she knew that her choice would set the seasons in motion.