When a dog's behavior changes, it may be due to underlying illness or pain such as from arthritis, veterinarian Alex Herman writes in response to an owner's concerns about a listless pet. A complete physical exam by a veterinarian along with blood tests and other diagnostics will help determine if illness is present, according to Dr. Herman. Medication, changes in the dog's environment or behavioral consultation may be needed to help improve the pet's condition, Dr. Herman adds.