The myth that dogs have cleaner mouths than humans is fairly persistent, but one whiff of a canine's bad breath may be enough to convince you that it isn't true. halitosis is caused by bacteria, among other things.
A recent article in Knox News addressed the many causes of oral odor among man's best friends. Some of them are exclusive to dogs. These include the ingestion of certain items that most humans do not consider food, as well as the use of the tongue to clean hard-to-reach body parts.
Likewise, the source notes that gum disease, food allergies, metabolic problems, gastric illness and cancer can cause oral odor in dogs, although these are more common in humans.
Overall, dogs have dog breath for the same reason that humans have halitosis - microbes. The bacteria on the tongue, teeth and palate live off of the food we eat. As they digest sugars and proteins, these microorganisms give off foul-smelling compounds that our noses recognize as bad breath.
To combat this condition, humans may consider using what canines cannot - oral care probiotics, which replace harmful and odiferous oral bacteria with less offending varieties.