I have to say sorry to you also then Herc as I didn't make myself clear.
Firstly, you said 'You only need to look at the incidents of dogs snapping at their owners to see that this is far more common than a dog laying its life on teh line'.......well it would be as there are far more irresponsible owners for dogs to bite than intruders to lay down their lives for.
Second, I know there are lots of people bitten by dogs. I work in the emergency services and I've seen it but the dog gets the blame too often. It is the owners who are at fault most of the time. Some family pets are pampered to such a degree that they understand their place to be Alpha. Foodbowl snapping is very common. These are animals and it's in their blueprint. watch any nature programme, who eats first? The alpha and the lower ranks get whats left. Dogs work the same way and foodbowl snapping is the dog saying 'I'm aplha, I'll eat and reprimand you if you touch before I'm finished'.
Biting children is not the dogs fault either, again the owner. As the owner of a dog you should be aware of the potential dangers around children. Why is any dog left alone with children, it's beyond me.
Again, nature programmes, what do you always see?....the little ones growing up and fighting for the next available place in the pack. They know they will not get the 'alpha' as that's taken but the next spot is up for the taking. A dog will see a child as a competitor.
Giving a dog a toy and allowing it to become completely possessive about it's toy is another reason dogs bite. It's his toy, you've instilled that in him and when you take it(as many bitten children have learned) you are getting him to best you, i.e whoever gets it is the best and therefore worthy of the next position available in the pack.
This post is way too long already so I'll stop but I stand firm in my conviction that dogs bite because people teach them (albeit unknowingly) to.