Because a dog's teeth are his only arms. Most dog bites occur on or near the dog owner's property. Your seemingly innocent actions may cause a dog to bite.
Many bites occur when a dog is playing with a person.
A dog who is sick or injured may bite anyone who comes near. Teasing, harassing, or confusing a dog can cause him to bite. A dog that is frightened or cornered may bite. A confined dog in a fenced yard or chained up or very close to his home may bite to protect his home territory. A mother dog with pups may bite.