Knowing this gives us an insight into how to approach the problem behavior. One way is to ensure the dog won't get scared, no matter the circumstance. The other is to prevent the dog from becoming aggressive if it does get scared. Most people address the former but not the latter. Early socialization is a key ingredient to preventing fear aggression, as well as a proper breeding line of dogs.
Fearful parents produce fearful pups.
Boldness is inherited to some degree. By socializing your dog as much as possible, you are showing him not to become afraid, no matter the circumstances. This entails taking your young dog with you into lots of different environments and exposing him to lots of different people, dogs, noises, sights, etc. Familiarity produces calmness. The first time you jump out of an airplane at 5,000 feet, you're petrified.