Is it love, or is it lust?
Or is it because she allows him to be free from his responsabilities?
She manipulated him. There's no way this manga's gonna end well. She just needed someone to escape her mother, and this guy ended up beeing her victim.
That's my take on it.
Maybe the author has other plans in mind. But this is one of the possibilities.
Just for clarification, I'm not saying this guy's a victim. As an adult, you're supposed to resist your urges and bear your burdens. It doesnt change the fact that he is her victim. If what I think is true, he is the victim of her schemes, althought he shouldnt have fallen for it.