Dogs who have respiratory issues will pant more. If your dog has pneumonia, they will not be able to breathe as easily, causing them not to get enough oxygen to their body. This will cause them to pant in order to take in more oxygen for their body. Pneumonia can be a very serious problem in dogs if not treated.
Lung tumors can also cause your dog to pant. As your dog gets older, they can develop cancerous growths. Sometimes these growths happen inside your dog's chest. These masses cause problems with your dog's lungs fully expanding, leading to your dog to pant more. If your dog is panting, it is best to have your veterinarian examine your dog for any respiratory problems. Your vet will most likely take x-rays to see what all is going on in the lungs. They can assess the heart and lungs with an x-ray and help you figure out why your dog is panting.
Dogs who are panting more could be trying to cool off, but there may also be an underlying medical problem that you need to get checked out. If your dog does not stop panting after you get them to a cool area and keep them calm, it is best to have them seen by a veterinarian. These conditions can be life-threatening if left untreated. The early you start treatment on your dog, the better the outcome, and the quicker your dog gets back to their normal life.