It's sad people don't understand how dogs think. We expect them to think like we do, but that's projecting a LOT of human characteristics onto another intelligent but totally separate species. It's unwarranted and it causes a lot of needless tragedies.
Ok, so you have a dog. He's big, but always been a "gentle ben", loves your little kids to pieces, sleeps on the floor next to their bed every night. A perfect family dog. You and the Mrs. are going out one evening, so you call grandma to babysit. She comes over, and everything is peachy keen until you are tying your necktie and hear a commotion downstairs. You run downstairs and ***!! Your "gentle ben" is ripping grandma's throat out! You manage to pull him off, but the damage is done. Soon the cops show up, and the dog menaces them too. Fido goes down in a hail of gunfire. *** just happened to your "perfect family dog"?
Ok, now from the dog's point of view:
I love my family, especially the little ones. Nobody better even think of hurting them. It's night, and now suddenly there's a stranger in the house... think I've seen her before but she smells funny... different than the family, anyway. Better keep an eye on her. Whoa, dad just left the room, leaving the kids with this stranger and only me for protection - condition orange! Suddenly she yells something and lunges for one of the kids, MY KIDS! CONDITION RED! I fend off the attack, suddenly dad is back in the room. It's ok, dad, I took care of the threat, see she's down now. Better not threaten my kids or this family again, lady! What's that noise outside? A strange car, two aggressive looking guys headed this way... they smell like testosterone and adrenaline, OMG the threat's not over yet... snarl a warning at them to keep away from my family! What's that black thing he's taking out off of his belt? he better not come any closer or I'll <BANG> (silence)