Remember that article I wrote about using positive reinforcement to change behavior when dealing with aggression? Well, obviously the scenario in it was hypothetical (the part about the snakes). That's not to say that it won't work, because it definitely would. However, since it was hypothetical I wanted to give you a real story using classical conditioning with a semi-feral cat.
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 20 years experience as a dog trainer I have found that aggression is usually caused by one or more of a few things: