When I assess a dog for aggression (what ever type it is) I always recommend a visit to the vet if I notice any physical problem .Once a Shar pei displayed a very common problem in the eyes. condition is known as entropion..... it is easy enough to correct if treated properly. I have seen dogs of a year old with this annoying problem . The eye lashes wrinkle inside and they are touching the eye constantly and as a result the dog becomes grumpy , irritable and aggressive . Once the entropion is corrected the aggression has to be corrected because now we have a case of pain induced aggression. Similarly I have encountered cases of dogs with mild to extreme hip displacement. The same procedure must be followed The physical complaint must be treated then the pain induced aggression can be dealt with.
Dogs that have been mistreated or abused by humans either out of ignorance or nastiness can present with severe cases of Pain Induced aggression .It is learned behaviour and is a response to self defence response to the mistreatment.. Physical punishment to an animal can induce aggression as a result of responding to aggression with aggression.