What do you call it when a dog seeks out a situation in which he goes into defense?

Example: Roman just hit the one year mark. We have llamas. Llamas are aggressive towards dogs, I get the feeling they are somewhat more like cows in that sense. They will try to stomp a dog. Roman learned at a young age to jump into their pen to try to chase them. They chased him and got him a couple times. At around six months or so he started barking at them with the hair on his back up. Deep barks, it could only have been defense. Now, at 12 months he will bite them if they kick at him or run past him. If they face him he keeps his distance. Maybe it's the breed, or maybe my dog is just a poor example, but he doesn't have the nerves of, say a good cur or catahoula. Anyways he loves nothing more than to bark at them, bristling and tail wagging (???). He looks forward to it. When they charge he runs from them but is right back at it. Now, I thought defense was when a dog is under stress, so shouldn't he want to get away from them, if they put him into defense? My own theory is that his prey drive is saying "got get 'em" and his prey drive overrides his fear of them, but he is still scared of them. But what do I know. It's mixed together kind of oddly and I am curious of what to make of this any where it might be seen in elsewhere in the dog world.