As Rooster said there are many reasons, and you should see a vet. And while at the vet make sure they check for pancreatitis. Just some highlights as to some of the more common causes, diet, intestinal infection, not enough good bacteria in their system, and colitis. Definitely get to a vet. In the mean time feed you dog boneless, skinless boiled chicken breast, or LEAN ground beef with either rice or boiled potatoes. You can add a spoonful pumpkin puree. You can get that at the grocery store for a much better price than pet supply stores. Just make sure you are getting 100% pumpkin and NOT pumpkin pie filling.
Of course it's curable, but you need to find the cause of it first! Might just be a diet issue or it might be something more serious. Either way, chronic diarrhea will eventually lead to dehydration. Take your dog to the vet and have him checked to find the cause so he can be treated appropriately.
Diarrhea is a symptom of many things. If it's persistent then it is a serious problem.
It could be caused from anything from food intolerance to illness. Priority would be to take the dog to a vet to have it checked out at this point. ASAP
Take your dog to the Vet as it could be caused by many different things. Good article here about it and what you should do.
Chronic diarrhea is a change in the frequency, consistency and volume of the dog's feces for more than three weeks. Starting in the small or large intestine, diarrhea can either be secretory (where it is very watery) or osmotic (not watery), and is due to various reasons, including diet, disease, or infection. Poor digestion, dietary, metabolic disorders, small and large intestinal abnormalities are causes of diarrhea. And yes, it is curable. Give the dog a small meal of boiled white meat chicken (no bones or skin) and white rice. This can be the dog's diet until the stool consistency returns to normal. If the diarrhea continues for more than 24 hours or your dog's condition worsens at any time, call your vet immediately.