Often one of the most common reasons why animals show aggression is due to fear, however there can be other reasons too such as frustration. Dogs do not show aggression because they are trying to be naughty or stubborn, they often have very good reasons. If your dog is showing aggression ask yourself why s/he maybe doing this in terms of emotions such as anxiety, fear, frustration, panic, play... Don't think in terms of labels such as naughty or dominant.