Separation anxiety is describes the condition that causes stress and un-desirable behaviour from a dog while away from its handler. It can manifest as soon as the dog is left alone in destructive, disruptive or agitated behaviour.
Typically it occurs when the dog has formed a hyper-attachment to their owner or the dog feels dependent on human company. Dogs with separation anxiety when separated for the owner even for a short time panics can cause damage to furniture, carpets and to themselves. They also may bark constantly until someone comes back to make them feel comfortable again.

It’s not fully documented why Separation anxiety is caused, but it could be caused due to changes in environment making the dog anxious and unsure of its surroundings so they need more reassurance from its owner. It may be that during house training and obedience training we as trainers unknowingly create a dependency as we are always there to correct undesirable behaviours and reward good behaviour, creating a strong dependant bond with the dog. Owners also unknowingly may reward the undesirable behaviour by for example letting them out of a create or giving cuddles due to them whimpering, barking or scratching the floor, reinforcing the behaviour.
There are many ways to help prevent separation anxiety just a few to mention:
When still a puppy when the brain is still developing it is essential that they can be left to explore in a safe area away from the handler so they can develop their own independence and help their self confidence grow. Making sure they only stray less than a few feet away may inhibit their feeling of freedom and reinforce a stronger dependant bond with the handler.
Constantly comforting a crying or disruptive dog reinforces the unwanted behaviour, so as long as they are in a safe place and in no danger of hurting their self they can be left until the crying and unwanted behaviour has stopped. Give attention when they got quiet reinforces the correct behaviour and also let s the dog know that they have no need to worry about you not returning.

Obedience training and discipline help the dog know what is expected of them and in time the good behaviour become habit. Consistently reinforcing good behaviour develops a good relationship with the handler and a trusted bond. Using the bond and practicing sit and stay while you go out of sight helps the dog trust that you will be back and there is no need to worry or get stressed while left for longer periods of time.