The myth varies wildly by who retells it but the pg version is he tricked her into eating a pomegranate seed which somehow was part of his domain, indebting her to him. She wasn't interested in him as a potential husband at first because he was the loner who was never on olympus while other god/heroes were making a name of themselves.

He may have been one of the most poweful gods but his reputation was that of a basement dweller without any great deeds or cool stories attached to him, since he spent most of his free time keeping all the dead people, ya know, dead.