Bad dog?

You see hundreds of dogs as a mailman. Some of them just look on or approach excitedly. Most of them bark out of fear or a sense of duty, but rarely, you see one that actually looks like it wants to kill you. That attempts to leap over the fence and crashes into it, barking and growling with the kind of aggression you don't hear from dogs in happy families.

I wonder if a dog that acts like that is happy.