What's the logic behind this relationship?

I know a guy (in his late 30s), who's been in love with this one particular female for years. He's always talking about her, almost bordering on obsession. He's convinced that they're gonna get married, have kids, and live happily ever after.

Meanwhile, the girl he's madly in love with has repeatedly stated that she hates him and wishes he was dead. She tried cutting off all contact with him, changing her number, and ignoring him at all costs. Despite all that, he still continues to send her large sums of cash, love letters, and roses in the mail. And she still hates him. He bought her a brand new car, and still...hates him. Clearly, he can't but her affection.

But how could someone be obsessed with a person who's shown clear hatred for them?