I frequently have this question asked of me. There are many and varied reasons for changes in your dog's behaviour. First of all we need to remember that dogs are intelligent, thinking beings. They are not programmable robots; training is not a one-time, set-and-forget activity. Training is an ongoing, lifelong interaction between you and your dog. It can be a conversation of "Yes, I like that behaviour, let's play with it" and the occasional "No, I don't like that one, let's not do it". Some changes are good, like when a pup learns to retrieve a toy and you turn it into a game, or when your dog understands and remembers a set boundary. However some changes are not so good and we need to be aware of the triggers and of how to assist our dogs with their choices.
The most common changes are what I call Developmental Changes. These happen as our pups are maturing from puppyhood into adolescent and adult dogs. Most commonly these changes occur at around 6 months, 12 months and 18 months of age. At 6 months the clingy little puppy suddenly matures into a more confident young dog and begins to exercise some independence. This is when I often get anxious owners wondering why their good little puppy no longer comes back when called. At 12 months, the adolescent pup is maturing into an adult dog. They might regress with their obedience training and become more stubborn and wilful. As they mature, dogs will try to exercise their independence just like teenage humans, and will push back against discipline and rules. This can also happen at the 18 month mark. This happened with my Koolie, Chui. When she was around 18 months of age we were testing for her obedience skills as part of the process of me becoming a qualified dog trainer. The first task was to ask her to do the most basic obedience task: Sit. She looked at me, and quite plainly responded with "Make me!". After a few attempts at this with her refusing to comply (and prior to this she was exceptionally compliant) we realised what was going on. The Senior Trainer who was testing us recommended we delay the test for a week, and in that time I go back to the basics and work Chui through every exercise to show her that the rules we had set in place still applied, regardless of what she thought her status was. We passed the test the following week with minimal issues and error. However, if I don't reinforce her training Chui will sometimes cheekily refuse to do as asked. She is a smart dog and is constantly thinking about our relationship and her choices.
Another factor affecting behaviours can be illness or injury. I recently had a call from a very worried client. Her dog had suddenly begun to show aggression and wouldn't allow her to handle him. However, if she sat quietly he would come to her for pats and attention. My first thought was that he might be unwell. She has since reported that the Vet thinks there might be some joint pain and now that her dog is on pain relief he is back to his normal, lovely self. Just as we get grumpy when we feel unwell, so do dogs. They can't tell us what is going on. We have to work it out.
Again with my Koolie, Chui, I had a similar situation. Instead of aggression though, she was much quieter than usual and slept a lot more. At first I thought it was due to her age as she is nearly 12 years old, but there was something slightly 'off' about her. She had broken a tooth and the nerve was exposed (yes, I winced too) so off to the Vet and after having the teeth attended to and some medication she is back to her usual goofy, chatty self.
Changes in routine can generate behavioural change as well. Our dogs are happiest when their lives are consistent and predictable. The more sensitive a dog is, the more likely they are to react to changes, even small ones.
Obvious upheavals such as moving house are going to make everyone feel unsettled, but even minor changes can worry our dogs. These can be things such as a family member moving out, a baby arriving, or a new partner moving in. Sudden influxes of visitors over the festive season can upset a dog. House guests can create change, especially if they spoil your dog! If you go away on a holiday and either kennel your dog or leave them with a pet sitter, these changes can precipitate behavioural change.
Counteracting these changes can be quite straightforward. In the case of illness or injury, Veterinary care should help your dog return to normal. In addition, if you already have done some training and have set rules and boundaries, you can show your dog that nothing is new, that you still have the same expectations of them as before.
By reinforcing the rules you are showing your dog that they can still rely on you under these new circumstances. They also have something constant running through all the changes; the rules and the obedience exercises still apply. The rewards will still come and your love and attention remains.