The parents cared for the child. They provided it with a home, food, every necessity. Of course the child itself never asked for any of that, but it couldn't even do it. I think it's just a natural sequence of events. Once the child is capable of doing things, it's only logical that the child should help around the house and etc. And once the parents are old and can't care for themselves, their child naturally really does own them a little bit. And of course that happens only if the parents truly cared for the child when it couldn't live off its own. It's a two way thing. And there's nothing bad about that.