How cute is Bella the maltese?!? She is a very smart dog and picked up everything quickly, as did her awesome owners. Very quickly we fixed her getting overly excited at the front door (someone passing out flyers actually rang the doorbell during our session) and we set the ground work to erase her separation anxiety. She and her owners learned how to separate while being in the house together without physical barriers. This is Bella chilling out in her crate while her owners were two rooms away out view!
Separation anxiety starts long before the owner starts to leave the house. Typically, it has to do with the relationship between the dog and it’s owner. Either the dog thinks it is in charge and supposed to “take care” of it’s owner, or the dog and human have a kind of co-dependent 
relationship and the dog doesn’t “know” how to be alone.
Put yourself in your dog’s mind. It thinks it supposed to keep you safe, and then you leave and lock inside the house. This is why dogs with separation anxiety can have a meltdown when you leave, causing them to bark (come back!) or try to dig out through the wall (I’m coming to get you and keep you safe.) The same goes for dogs that don’t know how to be alone. It can be overwhelming for them.
The key is to have a healty relationship with your dog. Just like a healthy relationship with a human, whether it be romantic, friendship or family, you want to share just a portion of your time/life with them. You don’t want to try to become “one” nor do you want there to be too much distance. Think of a the marriage symbol – you want to share the middle that overlaps in the ring. Too much or too little isn’t a good, healthy relationship.
