Fear Aggression is a defence mechanism in the dog's mind and it is a learned behaviour. Similar to a human mind the dog's mind has four elemental steps when faced with a situation that causes him fear ie fight , flight , avoidance and submission. When the dog is on the leash he loses the option for flight so his predisposition for fight is the only course of action . If the dog is off the leash he will also be ready to snap as as a mechanism of defence and will be less fearful as he always has the option of flight . In order to help the dog we have to show him that he can go through the situation with no harm . His fearful reaction to the stimulus ( whether it is particular noises or objects or dogs and humans ) has primarily two causes . Firstly it could be genetic. A dog may be born with what I call " the shy gene", This gene is inherited and in the situation where the dog is a young pup and has not yet been properly socialised the fear gets deeply entranched into his mind and fear aggression is the result. Fear aggression can also be the result of a very bad experience involving humans or other dogs . With humans sometimes the lack of knowledge about how to properly correct bad behaviour can cause the human to act in an aggressive manner or the behaviour may simply be plain and intentional abuse. Fear aggressive behaviour towards other dogs is the result of lack of proper socialisation or could be as a result of a very traumatic experience with an aggressive dog . Sometimes inadvertently after the traumatic episode the owner will nurture the fear without him/her knowing it .
In any case the important thing is to help to recover the dog's social skills so that he can enjoy the presence of other dogs or humans . Fear is real and a horrible emotion for any dog to have to live with. It is our responsibility to rehabilitate the dog to improve his level of well being . Also as an aggressive dog may display behaviour that has legal consequence for him and his owner it is something that requires to be addressed as quickly as possible after it presents itself as a problematic condition.