Think of it from a dog's perspective:
"Great. There's a dog running towards us. My human insists on tying herself to me, even though she's far from able to keep up with me, so running away is out. I've never seen her bite anything other than food. And she tends to talk to other dogs in that squeaky oh-aren't-you-cute voice, so she'll never convince this idiot to leave us alone. I guess I'm going to have to defend both of us. Maybe if I act really scary - the stupid mutt will leave us alone"