Aggression is one way that a dog can communicate how they feel - all dogs are capable of showing aggressive behaviour although some display this more readily than others. There are many types of dog aggression including:
Understanding the cause of the aggressive behaviour is the first step. Usually the problem comes down to the dog feeling insecure, anxious and fearful. Even dogs that otherwise appear confident, bold and happy can actually be displaying small signs of feeling uneasy but these are often missed by owners until it's too late.
When dealing with aggression, a dogs gender, age, health, hormones, previous experiences and genetics all play a role in the dogs behaviour. Each dog is different which means training can be quick for some while others may take allot longer to train.