There are several things that can cause coughing some much more serious than others. Allergies to pollen etc usually accompanied with watery eyes and sneezing much like humans, kennel cough a persistent cough that is contagious amongst dogs, heartworms unless your dog is on preventative should be tested to rule this out and placed on a preventative and congestive heart failure. I would have a check up done by the vet to determine what exactly is causing it