Like humans, dogs produce tears for a variety of reasons besides just lubricating the eye.
Tear stains--the brown, streaky stains in the corners of and under some dogs' eyes--are caused by pigment, yeast and bacteria buildup which, in turn, are commonly caused by excessive hair in eyes, allergies, ear infections, poor food quality, and/or blocked ducts. These stains are most obvious on light-colored dogs, and some breeds are more prone to excessive tearing than others including poodles, Maltese and shih tzus.
First and foremost, make sure to regularly trim long hairs near your dog's eyes. Doing so will help prevent tears and watering, as longer hair is more likely to trap bacteria and yeast which cause eye irritation.
Change the pH
Changing the pH of your dog's tears to a more alkaline nature can reduce staining. The change in alkalinity makes it difficult for yeast and bacteria to thrive, causing less eye irritation and fewer stains. Add a teaspoon of cider vinegar to your dog's water bowl daily to keep tear stains down.
Improve Food/Water Quality
Provide your dog with a high-quality dog food and water. If you need a recommendation, check with your vet or an employee at your local pet store. The high-quality dog food can help prevent tear stains in the first place, plus it encourages a strong immune system, to help prevent other problems from developing which may make tear stains worse. For the water, consider using bottled or filtered water. Tap water contains many minerals that can cause the tear stains to develop. Another option is to use stainless steel bowls for your dog's food and water. Plastic bowls can hold bacteria that cause fur discoloration.
Make a Lightening Paste
Mix 1 tablespoon Milk of Magnesia with 1 tablespoon hydrogen peroxide. Hydrogen peroxide blanches the dark brown stains that white dogs develop near their tear ducts. Add enough cornstarch to make a thick paste -- about 1 tablespoon. Spread the solution on your dog's tear stains, taking care to avoid getting it in his eyes. Let it set for several hours and wash with a wet wash cloth dipped in warm water. Comb out the hair near your dog's eyes. This not only smooths hair away from your dog's eyes, helping prevent eye irritation and watering, but also helps break up residue collecting near the eyes. Repeat the process on a daily basis until stains fade.
Ease Your Fears
While over-the-counter home remedies for tearing are available in most pet stores, be sure to have your vet first determine the tearing is not caused by a greater health problem. Sometimes tearing is caused by an injury, embedded object, infection, ingrown eyelash or other condition requiring veterinary attention.