Becca's right.
Case in point, two of my neighbor homes across the street each have two large black dogs (probably lab-rottweiler or lab-doberman mixes) who will 'guard' their yards ferociously -- vociferously, at least -- whenever anyone approaches. (I park in the carport in between the two houses sometimes.) They are so loud and vicious sounding that for a long time I was afraid to even dare getting within biting range.
After a while I noticed things like wagging tails and even, in the biggest dog's mouth, a chewed up tennis ball, as the growling and jumping continued.   The worst thing these guard dogs would probably do would be to lick an intruder to death.