Bull Terriers need both physical and mental stimulation provided by their owners by daily exercise. Exercises give a dog a natural outlet for energy. They create strong bodies and good muscle tone for the pet. Lack of sufficient exercise, will result in boredom and behavioral problems. A well-known yard or a house is often boring for a dog. Even if the dog is in a big yard, few exercise on their own, so it will presumably sleep all day without the interaction of exercise by the owner.
Exercising with your dog creates a human-dog bond. It is a great way to show that you care about your dog and make it happy. Tired dogs are the happiest ones.