There are several reasons why a dog can sneeze frequently, including allergies. Like humans, dogs can develop allergies to common products.
Dogs can also sneeze repeatedly when they are suffering from nasal congestion. The nasal congestion can occur with nasal discharge, watery eyes and coughing, according to WebMD.
Another cause of constant sneezing is a foreign object in the nose. The sneezing may be followed by nosebleeds. If the dog continues sneezing for several days or has blood draining from the nose, owners should consult a veterinarian as an object may need removed or the dog may need treated with medications.