Dogs become vulnerable to toxins and toxicants and loose their natural immune protection.
Nutritionally inadequate diets lead to defective adrenal glands unable to manufacture adequate cortisol, a hormone vital for health and resistance
to infectious disease
They will never tell you about this.
They are speechless victims of human toxic environment. 
They just beg for your food...

Cancer: The chemicals & preservatives used in many commercial dog foods are known to cause cancers.The same cancer-causing chemicals and preservatives have been banned from use in human foods because of these known problems...

Heart Disease: The high proportions of cheap unhealthy fats found in many commercial dog foods and lack of more expensive omega-3 essential fatty acids (EFA) cause arteriosclerosis and arterial blockages resulting in death by heart attack.

Kidney Disease: An 11-year study of acute renal failure in dogs concluded 45% of cases are caused by toxins and 35% by infectious disease.