Separation anxiety can appear in dogs that have not yet been properly socialized, dogs that were transferred from one home to another, and dogs that are naturally nervous. Your dog is a social, pack animal who relies on others (you) for human protection and security.
Dogs that lack confidence, due to socialization issues, understanding of expected behavior, or possible maltreatment previously (adopted dogs), are more likely to exhibit behaviors associated with separation stress. Treating separation anxiety in dogs is easy if you take the proper precautions.
There is a lot of debate on the reason for separation stress. Some think it's a lack of confidence that causes your dog to rely too much on their own humans. In cases like this, you have to teach your pet to accept how you're departing, and it's really going to be fine.
Make your dog realize that your departures and arrivals are nothing to get excited over. Consequently, you have to exercise with your pet the"routine" of moving and coming. Exercise is thought to relieve stress in both dogs and humans alike.
A very long walk, run, or play session prior to leaving can go a very long way in helping reduce the strain and anxiety of your pet. If a dog has another companion to keep him company as long as you are gone, then it might help alleviate the boredom and tension caused while you are gone.
Of course, this is a mere "one" option. There are many other things to take into consideration before adding another dog to your property. This only provides basic information about separation stress. Your veterinarian is the best source of health info. Ask your vet to learn more concerning separation stress and its prevention.