A animal psychologist and zoologist says poor social skills drive dogs to attack other animals and people.
And in the case of attacking animals like sheep, once they start they find it hard to stop.
Two dog owners who have had themselves and pets attacked spoke out about Hawke's Bay's roaming dog problem at the weekend - within hours of their story being told a horse, goat and 49 sheep were killed by two dogs - one which is still on the loose - in Bridge Pa, Hastings.
Animal behavioural expert Mark Vette said the dogs involved in the Bridge Pa attacks were likely to have been poorly "socialised" and not been exposed to other animals.
"The ancestral dog was a predator, so that predisposition to hunt is there, even though what happened over the 40,000 year evolution of the dog is that they became scavengers around settlements," Vette said.
Vette said the dogs who attacked the sheep in Bridge Pa were unlikely to have been socialised with livestock and the fast movement of the sheep would have triggered their hunting instinct.
If there is a flock of sheep, their furtive movements continue to attract the attacking dogs, who quickly move on to their next victim.
Sheep that died at Bridge Pa at the weekend were found in groupings which suggested they survived the initial attack, but died soon after.
Vette said that given the attacking dogs were most likely fed by their owners, they would not associate sheep with food, which is why the sheep were not eaten.
"There's no relation between the hunt and eating - dogs who usually attack livestock are not trained hunting dogs, so when the sheep run their instinct is to chase them."
Hawke's Bay's 'roaming dog problem': Victims speak out
Up to 49 sheep killed in frenzied dog attack
Vette said the two to four month period of a dog's life was paramount in terms of social skills and it was important to introduce them to different animals and people during that time.
"We call it the formulative period for socialisation. If you're rearing your dog properly, you need to cross-foster them on to different species in that early period.
"So that means sheep, cats, chickens... they need exposure to them and grow up thinking that their family - which of course is much easier to do when you're on a farm block or rural block."
Although Vette said almost every breed could be socially trained to accept other species, some breeds had a "higher level of predatory drive".
"We see these traits in the hunting breeds like your Pointers and Weimaraners, German Shepherds ... most of those breeds have the predisposition to be predatory if you don't socialise them properly."
Vette said bull terrier breeds also posed similar behaviour if they were not trained or socialised correctly and had an element of "gaminess".
"That means they have a really high drive when they're aroused to hunt and that can even redirect dogs like pitbulls into fighting with other dogs.
"That trait isn't desirable as they can lose their inhibition to submission. It's natural for when a dog shows submission for them to stop fighting, dogs have a highly sophisticated signalling system when it comes to dominance and sub ordinance."
While it was important to socialise dogs with other animals, it equally as crucial to socialise them with humans too.
"When you think of them in the wild, dogs grow up in a pack and become familiar with one another.
"So as an owner you need to grow them up with a very broad pack so they can see other people coming in, particularly different races, different ages and genders, as everyone behaves differently around dogs."
Vette and his team had been working on a project called "dog zen" which promotes the idea that if all dog owners trained their puppies within the vital 3-16 week period, it would transform dog aggression in New Zealand.
Helen Beattie, chief veterinary officer at the New Zealand Veterinary Association, also put dog aggression down to certain factors including hereditary factors, early rearing experience and the owners' attitudes, experience and reasons for owning a dog.
The owner of a dog that attacks stock is liable for the damage they cause and a fine of up to $3000. The dog can be destroyed on the spot.
The animal seized after the Bridge Pa attacks is not registered, but Hastings District Council contractors believe they know the address the animal came from.
Sue Dixon, who had sheep killed at the weekend, said Hawke's Bay had a "major problem" with roaming dogs. She said attacks were becoming more frequent across the region and something needed to be done.
Meanwhile, the 20-year-old owner of a dog that attacked a nurse visiting a Hastings home will be sentenced next month after she admitted a charge of owning a dog that caused injury.
Pounamu Morunga-Cooper's mastiff labrador cross, Smokey, attacked the nurse during a home visit in Hastings on March 24.
X-rays showed the woman suffered serious deep puncture wounds and bruising to her right buttock, the biggest being 2cm long and 2.5cm deep.
A judge last week ordered that Smokey be put down.
Council is still looking for a second dog believed to be involved in the weekend's attacks at Bridge Pa. It's understood the second dog is not owned of the dog already impounded.
Council declined Hawke's Bay Today's request to photograph the impounded dog, saying the owner had not consented and council "does not want to compromise the evidence".