As a dog trainer I deal with many behavior problems. Aggression is one of the most common reasons I get called to someone's home, whether it is aggression directed towards a person or another animal (usually other dogs). In my 25+ years’ experience as a dog trainer I have found that aggression is usually caused by one or more of a few things:
1) under-socialization/fear
2) a traumatic experience when the dog had little to no "padding" of good experiences
3) mishandling of the situation by the owner
4) genetics