Why is my Boston Terrier dog sneezing non-stop? What are the causes?

Thanks for using Petco Pet Education Center, formerly Petcoach! The good news is that in most cases this is something benign and transient with environmental allergies being the most common cause. Other possible causes include a possible foreign body stuck in the nose or another part of the upper respiratory system. I would recommend that you have Sadie evaluated by a veterinarian for further assessment and treatment. If based on further history and physical examination it seems like allergies are more likely, medications such as Benadryl to be used on an as needed basis would be recommended. If a foreign body is suspected, further imaging such as with a CT scan may be pursued to confirm that. Hopefully, this is not anything serious. Good luck and have a great day!