A dog that is not active or energetic may be due to illness, tiredness, age, physical discomfort, etc. Generally, colds, canine distemper, parvovirus, coronavirus and other diseases can cause dogs to become mentally ill. reaction, it is best to take the dog to the hospital for examination. If there are no problems with the dog's health, it may be that the dog is tired from playing or is older. Generally, dogs will start to become more stable when they are five years old, no longer as lively and active as they were when they were children, and they like to sleep more.