According to research conducted by Evan MacLean at the University of Arizona and published in the journal Frontiers in Psychology they found that the hormones oxytocin and vasopressin may be linked to aggression in dogs. Both hormones are also found in humans.
Dogs that tested to be more aggressive had higher levels of vasopressin. What is interesting is that further research of dogs bred to be assistance dogs who are bred specifically to be non-aggressive, had higher levels of oxytocin and higher oxytocin-to-vasopressin ratios. What this means is that oxytocin may help inhibit aggression.
Researchers also found that experience can influence the level of vasopressin in a dog. Often aggression results from a traumatic experience which alters the hormone levels resulting in a form of PTSD. On the flip side, positive experiences such as socialization with people and other animals in a non-threatening manner can raise the oxytocin levels.
The good news is that in humans, they are already using hormone therapies to help people with autism, schizophrenia and other problems such as PTSD. Perhaps this will lead to therapies for dogs that are extremely aggressive.