It is about the inherent nature of each individual animal.

When I lived overseas a street dog adopted us. Came into our compound while she was still rather young. Must've been some sort of mastiff mix because she had jaws that could crush a Sherman tank. I was her favorite. We had a game where she'd take my hand (a quite small thin hand) in her mouth and I'd whack her gently with a rolled-up newspaper and we'd start all over again. She knew I knew she could take my arm off any time she might choose to and that she would always choose not to.

But I would not ever under any circumstances have told anyone else to let their child play that game with that dog. I might well have been a moron, myself, to have trusted her absolutely as I did.

You can't make warranties about the nature of another creature. You can try to choose sensibly and train intelligently and even so anything can happen and sometimes, unfortunately, it does.