Separation Anxiety
Separation anxiety is a phobia — a fear of being left alone. It is treatable, but it needs to be handled delicately, and most dogs do not “get over it” with time.
It can occur for a number of reasons including, but not limited to, changes in family dynamic, moving to a new house, previously being left alone for way too long, *potential* genetic predisposition, unexpected scary experiences when they have been left alone (like thunder or fireworks), or having been through the shelter and rescue system. But none of this means that your dog cannot learn to be alone at home!
I highly recommend starting training right away. Here is a link to a self-paced online course for owners of dogs with separation anxiety. There are also options of working with certified separation anxiety trainers at more expense, but it is totally worth it if you can afford it.
Recommended Reading:
Be Right Back! How To Overcome Your Dog's Separation Anxiety And Regain Your Freedom by Julie Naismith
Be Right Back! Puppy Separation Anxiety Edition: Your Simple Step-by-step Guide to Raising an Easy-going, Independent, Happy-home-alone Puppy