Why does your dog sneeze and when should you take it to a veterinarian for sneezing?

Dogs sneeze because there is irritation inside their nasal cavity and they are trying to clear out the irritation.

If your dog sneezes on rare occasions this is normal and doesn't need a veterinarian's examination. However, if your dog is blowing out snot or blood, appears otherwise sick (not eating well, vomiting or diarrhea, coughing, laying around and not playing, etc), this would be a good reason to take your dog to a veterinarian.