Why does my dog have the runs?

There may be many reasons for diarrhea. It could be for something simple or extreme. For example, parvo, parasites, bacteria infection, or maybe he just ate something he should have. If the diarrhea is persistent it then it would be best to take him to the vet.

Possibly due to: Stress, Change in diet/food intolerance/eating spoiled food/ingest toxins/foreign object, Internal parasites, Bacterial or viral Infection, or possibly some other disease or infection. You may need to see the vet - depending how long it's going on, other symptoms, and what may have caused it (i.e. does the dog have preventative regular treatment& check ups for parasites, etc).