There are a few different reasons that a dog may hump the air. One reason is that they are excited and trying to release some energy. Another reason could be that they are trying to assert their dominance over another animal or person. Lastly, humping the air could also be a sign of an underlying medical condition such as an infection or hormone imbalance. If your dog is humping the air frequently, it is best to consult with a veterinarian to rule out any potential health issues.