Dogs pant when they are hot, but constant excessive panting can also be a symptom of an illness or a sign of stress. If your dog is panting excessively it may or may not be a reason to rush to the emergency vet, but is definitely something to monitor. Panting can be a symptom of lungworm or heartworm, both of which require medical treatment, or a sign of the dog's heart not beating efficiently enough to fully oxygenate the blood.
Evaluate your dog and if you have any doubt, book an appointment with your vet ASAP. Sometimes excessive dog panting is caused by obesity or stress. An obese dog may literally feel crushed by his weight, to learn more about helping your dog regain a healthy weight see our article on obese dogs.
Has your pet experienced any major changes recently? A new dog? a new family member? A new home? The answer to your dog panting problem may just be more quality time or allowing your dog to have a "safe place" to get away on his own, like a crate with a crate cover or a dog bed placed in a closet. Be cereful not too coddle or overly soothe a dog panting because of stress- rewarding anxious behavior may actually increase the panting. Some foster dogs have been known to pant for several days straight when placed into a new, uncertain, environment. If you've ruled out stress and medical causes for your dog panting, your dog may just be "a panter".
In investigating our panting dog and consulting with vets and other dog people we heard about several dogs who, even when relaxed, healthy, and cool just panted frequently. Apparently, some deep chested dogs may pant for no other reason than it's comfortable for them.