Agreed, there are literally hundreds of reasons why a dog might have diarrhea. "Dietary indiscretion" (ie eating garbage, underwear, or anything else that they shouldn't have) is probably the most common cause. But so are worms, giardia (a protozoan parasite which is of particular concern because it can be passed to humans), bacterial overgrowth, not enough bacteria in the gut, gastrointestinal obstruction/partial obstruction, bacterial infection of the GI tract (such as salmonella, e. coli, or campilobactor), pancreatic disease, other metabolic diseases, the list goes on. A little more information might help narrow down which causes are more likely and help people give you advice on what to do about it.