This is covered in the book FWIW. The argument that the author makes is that despite H. pylori being the primary cause of ulcers, it doesn't explain all cases.

This is covered in the book FWIW. The argument that the author makes is that despite H. pylori being the primary cause of ulcers, it doesn't explain all cases. He argues that even controlling for lifestyle variables (drinking, smoking, etc) that stress it self still causes a two to threefold increase in risk ulcers.