It's natural for a dog to bark. It's one way he expresses his feelings – everything from excitement to agitation. If you have an incessant barker, it's important to teach him an alternative behavior like the "quiet" command. If your dog is fearful or aggressive, though, the "quiet" command might not be the right fit for your pooch. Talk to your veterinarian, who may recommend a referral to a behaviorist or certified trainer for your pet.