Stephenie Meyer said this quote

That's certainly a problem. But that's not what I was thinking of. It's just that you are so soft, so fragile. I have to mind my actions every moment that we're together so that I don't hurt you. I could kill you quite easily, Bella, simply by accident. His voice had become just a soft murmur. He moved his icy palm to rest it against my cheek. If I was too hasty… if for one second I wasn't paying enough attention, I could reach out, meaning to touch your face, and crush your skull by mistake. You don't realize howincredibly breakable you are. I can never, never afford to lose any kind of control when I'm with you.