Why does my dogs breath smell like death? The main, most common cause of bad breath is the buildup of bacteria on your canines teeth. This is called plaque. Usually, a healthy mouth will contain bacteria in it. However, the growth of harmful bacteria (which is the plaque) is never a good sign. This produces an unpleasant odor. If not removed, your dogs will experience irritation and inflammation around the gums, called gingivitis, which is already the first sign of gum ...