Few dogs can resist chasing a squirrel that crosses their path, for example. Loud noises, such as a car backfiring, can trigger an otherwise calm pooch into bolting for cover. Even the sound of waves crashing along the shore can prompt a water-loving dog to a treacherous dip in the lake. The fact is, no matter how well you know your dog, you can't predict his or her actions when startled. And unless your dog is on a leash, he or she could run into a dangerous situation in a fraction of a second.
Some people are deathly afraid of dogs.
Seeing one with no leash literally makes their hearts race in fear. It doesn't matter how gentle your animal is, they are still terrified. Frankly, you can never make these people like your dog. But keeping it under visible control with a leash will help them tolerate its presence. Though your dog may be friendly and run up to other dogs in eagerness to play, not all dogs are so inclined.