There are many reasons why a dog might pant excessively. Panting is the main way dogs maintain a normal body temperature when it is hot outside. This evaporative process allows dogs to get rid of excess heat.
On the other hand, panting can be associated with many diseases. One condition that affects the adrenal glands called hyperadrenocorticism (also known as Cushing’s disease) can result in increased panting. Many respiratory and cardiac diseases can also cause dogs to pant. Obesity often leads to excess panting due to compression of the lungs from fatty tissue and an inability to cool off normally. Pain (due to conditions such as osteoarthritis and pancreatitis) can also cause increased panting. Dogs often pant when they are anxious or fearful (e.g., as a result of storm phobias or separation anxiety).
An examination by a veterinarian is the best way to determine if a dog’s panting is normal or abnormal.