Dogs are more likely to bite when
Poorly trained, socialized and maintained. Dogs
who do not know how to coexist with us, who do not get to experience and become comfortable with things in our world, dogs who are confused or allowed to be in positions where they may feel the need to bite are more
likely to bite. This includes dogs who are left unattended in yards as well as those who are expected to behave and act just like humans. Dogs are not humans and should not be expected to be just like us. However,
dogs need to learn positively how to live with us. Dogs need to be taught that we can take things from them. Many children are bitten when trying to take away a toy or when approaching an eating dog. Dogs are often
possessive of things. They need to learn to accept us being near when they are eating or chewing on a toy. This is all part of good training and socializing. Unnuetered males have the highest incidence of biting.
However, just being intact is not the cause of biting, there are often other underlying factors including poor training and management. Hormones can exacerbate some behaviors, especially those with a hormonal basis. However, neutering will not stop undesired behaviors: it is only one of many steps needed to work to resolve them. Training methods using prongs, shock collars and chokes can all lead to undesired behaviors and stress. Electric shock fences do the same this - create stress and aggressions.
Scared. Dogs react to things by either running (Flight) or trying to stop it (Fight). Dogs who are scared or anxious are more likely to bite. They do not have words to express what they are feeling and humans often do not understand dog body language. Often I see children wanting to run up and hug a scared dog to try and comfort it, this is very dangerous. The dog may become even more afraid and bite in order to try and stop the fearful event.
Lower tolerance. It is amazing what we humans expect a dog to put up with that is not natural for a dog to accept. We need to build tolerance in our dogs to things like noises, not chasing fast moving objects, having their body handled (tail grabbed, ears touched, mouths opened, etc.). A dog who will not tolerate things is more likely to bite. However, building tolerance is NO excurse for allowing a child to behave badly with a dog.
Feeling ill or hurt. A dog who is not feeling well or having a bad day will have a lower tolerance to things. Senior dogs often have more aches and pains that can go along with aging. A very tolerant young dog may lose tolerance as he ages due to discomfort. Again, dogs have no words and humans often ignore dog body language. Dogs with lower tolerances to things are more likely to bite. Sudden loss of tolerance should be brought to the attention of a vet as there could be a medical reason for it.
When overly excited. It never ceases to amaze me how many bites happen from play gone too far. Often it is directly the fault of the human not stopping play when it gets too rough or playing games that can teach biting and roughhousing with humans is a good thing. Or dogs who are not taught proper greeting techniques and get very jumpy and "hyper" (often out of confusion) when people enter the house. The dogs are not mean, just in need of training and better socializing. And the humans need to learn how to properly manage the dog. Over excitement and "hyper" behavior not only stems from poor training and socializing but also confusion and a dog not getting enough exercise, (physical and mental workouts.)
When they have puppies. Mother dogs can become very protective of their pups and may not want just everyone coming up and grabbing a pup.
When sleeping. There is a reason for the old saying "Let sleeping dogs lie." A tired dog is more likely to be less tolerant of things. Startling a dog awake can lead to a bite.