When we come to think of it, all wild animals of all sizes and species are aggressive. They are aggressive when protecting their territory, themselves or their offspring.
All beings that live in groups also use aggression to solve some of the social issues they face, including humans.
That been mentioned, every dog can be aggressive, regardless of their breed or size. The difference between one breed and another is the size of the damage the aggression can cause.
The most difficult training to invest in, is the one controlling the aggression of your dog, yet we hear all the time how people consider themselves trainers claiming they made a dog aggressive. The most the trainer probably did is make your dog so frightened and stressed that it became aggressive. We here tell you anyone can make a Chihuahua aggressive.
One of the main causes of aggression is Fear, therefore if you put the dog in situations where the dog had to fear his life, he will be aggressive. Insecure dogs also often show aggression.
Other causes can be Pain. If a dog is in pain it can show aggression, therefore we always recommend muzzling your dog at the vet if pain might be involved.
Dissatisfaction and Frustration are also a very important factor. If a dog receives enough exercise and mental stimulation, he is less likely to develop aggression. Yet, if the dog gets no chance to let out the energy, it will often turn into aggression. If you leave a dog locked up all day in an apartment, the dog can easily develop aggression.
Socialization plays a big role as well. It is important to socialize your dog to avoid aggression. Read about socializing your dog.
We would like to emphasize on our related advice to you; before you spend money on getting a ‘trainer’ to teach your dog aggression, invest in someone to train you how to control this aggression, if not then you put yourself, your dog and everyone around you in danger and create a fearful unsatisfied dog.
Read about what to do if you encounter a pack of stray dogs while walking here