What does the veterinary world believe to be the root cause of Cushing's? Stress or over-vaccinating of dogs?
Cushing's is a very expensive disease to "live with" - as a result, it would be nice for pet owners to have more substance to work with in terms of understanding what causes this condition in the first place.
All of your questions are good ones, and I wish I had the answers! You are correct that Cushing's disease is very common in dogs but extremely rare in people. We do not know why but this has been the case for many years, and it is highly unlikely that stress or over-vaccination plays a role in the canine disease.
Most of these dogs (and people) with Cushing's disease have a small pituitary tumor. In both dogs and people, these pituitary adenomas are monoclonal neoplasms, but why they develop remain unclear.
And finally, you are right about the cost. This is a very expensive disease, both in the diagnosis of the disease as well as the long-term treatment. All of the treatments for Cushing's disease in dogs, which include (1) medical (mitotane or trilostane), (2) surgery hypophysectomy (i.e., removal of the pituitary) or adrenalectomy (i.e., removal of one or both adrenal glands), or (3) pituitary radiation are extremely costly.
Hopefully, we will be able to make some progress in the pathogenesis and develop better and less expensive treatments in the near future.