Why does separation anxiety develop in dogs? Was it something I did?

Absolutely no! There was nothing you did or did not do as a dog owner/dog parent to cause separation distress in your dog. Rest assure of this. What we know from science is that mammals have types of attachment – these can be secure or insecure, and we know that they can be based on past traumas and life experiences. We know that separation anxiety is deeply related to the type of attachment your dog may have which is not a reflection of your love and care for them.