All animals, including humans and dogs, respond to the same powerful principles of learning.
One of these principles, classical conditioning, is responsible for dogs learning many things we don't mind them learning, but also countless things we never want them to learn.
In order for classical conditioning to occur, a dog must experience the pairing of something 'neutral' with something that already has meaning to the dog. For example, the sound of a collar being picked up is initially neutral to a dog: it doesn't cause the dog to feel any way in particular when they hear it. But then something changes; every time the dog hears that sound, it is paired with something that has meaning to the dog: fun walks. Pretty quickly, just the sound of the collar being picked up causes the dog to feel the same positive emotions they feel when actually out on a fun walk, causing them to bounce excitedly by the door. It's important to note here that unlike another form of learning we will discuss next time, the dog didn't actually have to do anything active in order for this learning to occur. This association that was created was totally involuntary on the dog's part.
Clearly, this sort of pairing isn't something we mind the dog learning. It actually plays to my advantage here as a dog owner, as I don't have to go and catch the dog in order to get them ready for a walk. As you can see in this video clip, the sound of the collar being picked up brings my dog, FigJam, to front door, as he anticipates a fun walk:
However, classical conditioning frequently results in involuntary pairings we don't want dogs to learn. Dogs who have serious fears or phobias, or who dislike specific events or things in their environment ('triggers') are experiencing these issues as a result of classical conditioning. For example, dogs who develop serious fears about toe nail trims do so when the previously neutral nail trimmer is paired with a frightening experience during nail trimming. Under similar conditions in the future, the mere appearance of the nail trimmer results in the dog feeling frightened, which causes them to try and escape.
Once such pairings happen, unless work is done to specifically address the issue, the dog's involuntary emotions when faced with the trigger won't change. They may likely even get worse. Resolving such issues takes concerted effort and, often, a considerable amount of time. Despite best efforts, even once a new, positive association has been created for the dog, such issues are at risk for relapse again in the future. This can happen, in part, because the memories about the previous events can never be fully erased from the dog's brain.
In light of this, when handling or training dogs it is important for owners and trainers to be very aware about any involuntary associations being created. Training or handling methods that cause dogs to feel fear or experience pain increase the likelihood of unwanted classical conditioning happening. If this occurs, classical conditioning can be used to change the dog's response to the trigger, creating a new, positive association instead.