When her mother found her, it was hell. However, Persephone was a fully grown Goddess, and her power had been getting stronger when her mother had held her in the Mortal Realm: Demeter could do nothing.
Every day, though, Demeter came back. She would throw a fit, wreak havoc on Hades' hard-earned calm. By this point, Zeus had threatened to take over for Hades until he could figure it out.
Persephone decided she would help; she wasn't about to just let Demeter treat Hades and the Souls like that.
"Mother," Persephone said, about a month after this had gone on. "I am not being held captive, and I wish you to leave Hades alone."
"You need to be working, Kore," Demeter said, her voice icy cold. It made Persephone want to wither; however, she stood tall.
"Fine. How about this: I will come to the Mortal Realm every year, but only for a few moons. Then, I will come back down here and be his Queen."
"Are you proposing to marry Hades - the man I've told you all your life to avoid?"
"It is not out of spite, mother. We love each other."
"Swear it, Kore. Swear it so I know you're not lying."
"I swear on the River Styx that I love him."