Because she ate the seed's of a pomegranate (the food of the dead) and Hades said she must stay in the underworld.
She could not then leave the Underworld, and would have to return for a portion of the year.
Kidnapping, most assume, Persephone did the eating of the food of the underworld part, which stuck her in the Underworld; guaranteeing she could never fully return to the world above: but she had to return thus becoming the goddess of spring.
When Demeter could not find Persephone, she grieved deeply and went searching for her daughter all over the world. During this time, the Earth became barren as Demeter neglected her duties as the goddess of agriculture. This led to a famine until Zeus intervened and negotiated for Persephone to return for part of the year, bringing about the cycle of the seasons.
When Persephone was kidnapped by Hades, Demeter was in mourning and all the crops withered or died. Zeus was worried whether mankind would survive or not, so he sent his son Hermes to make a deal with Hades. During her stay in the Underworld, Persephone eaten six pomegranates, and Hermes thought fast. The deal was that Persephone would stay in the Underworld for six months, and she would be free to roam the land of the living for the other six months of the year. All of the gods agreed, including Demeter, and that is why for six months, when Persephone is free, the crops grow tall and strong and the flowers are in bloom, and for the other months Demeter mourns and the grass shrivels and the crops die, creating the seasons.
In Greek myth, it is because of the abduction of Persephone that the Earth/we know the seasons: for Persephone also ate of the pomegranate in the Underworld and every year must return between Earth/Olympus and Underworld.
if he goes to the under world and back then through a 6 headed demon and a living whirl pool
When Persephone emerges from the underworld, it marks the arrival of spring and the return of life to the world. This event symbolizes rebirth, growth, and the cycle of the seasons in Greek mythology.
That could be either of these: Hephaestus - his wife was Aphrodite, and even though they stayed married (gods couldn't really get divorced) she conducted a very open and acknowledged affair with Ares. Hades - his wife was Persephone, who he admittedly kidnapped, but he truly loved her, and yet was robbed of her company for three-quarters of the year because her mother was Demeter and she forced Zeus to allow Persephone to return to the living world every year except during winter - that is also the myth explaining the changing seasons.
Well, one day, Persephone was picking flowers with her friends, when Hades spotted her. Immediately, he became entranced by her beauty, and he snatched her up in his ebony charriot, and dragged her down to the underworld. Her mother, Demeter, feeling the loss of her child, let the world's plants wither, and crops fail. Outraged, zues demanded that Hades relinquish Persephone. Unwilling to live without her, he fed her a pomegranite from the garden. Because she ate the food of the dead, she was technically deceased, meaning she must return to the underworld. So, about every six months, Persephone goes between the mortal world and the underworld. This explains the seasons. In the summer, Dememter is happy, she'll have her daughter for half a year, but, when she realizes her daughter must soon return, she lets the world begin to die, causing autumn. In the winter, she grieves, letting the world die and freeze, and when she realizes she shall soon return, she ready's the world for her daughter's homecoming by rejuvinating the undergrowth of the world.
Persephone made a deal with Orpheus to allow him to bring his wife, Eurydice, back from the underworld on the condition that he not look back at her until they were both safely back in the world of the living. However, Orpheus broke this condition, causing Eurydice to be lost to him forever.
In Greek mythology, Psyche was able to escape the underworld by completing a series of tasks assigned to her by Persephone, the queen of the underworld. She successfully completed these tasks, which included proving her worth and devotion to her beloved husband, Eros (Cupid). As a result, Psyche was allowed to return to the world of the living.
Persephone was bound to the underworld because she had eaten pomegranate seeds while there. Eating food in the underworld meant she had to stay there for a portion of each year, leading to the cycle of her spending part of the year as queen of the underworld with Hades and the rest above ground with her mother, Demeter.