Every once in a while there might be more than love in those sweet, puppy dog eyes. Occasionally, every pooch gets eye boogers. When should you worry about the buildup, and when should you just wipe it off?

During a dog's normal day, he's bound to get dust and debris in his eyes sometimes as he's running around the yard or diving headfirst into a pile of sofa cushions. Just like in humans, it's the canine tear ducts' job to flush the eye to wash away the material and clean out the eye. When that happens, the gunk typically accumulates in the corner of your pet's eye, creating a harmless blob that you can gently wipe away.

But if the gunk accumulates pretty regularly — or if it's thick and yellow — there could be a different, sometimes more serious, reason for your dog's eye discharge. Here's a look at some of the main causes of canine eye mucus.

AllergiesDogs can have allergies just like people. They especially may react to pollen or dust mites, just like we do. In addition to itching and scratching, allergy symptoms can include red, runny eyes with a clear, watery discharge. If you can't identify the source of your pet's allergy, call your vet before raiding your own medicine cabinet. Not all human allergy medications are safe to use on pets.

Conjunctivitis — Known simply as pink eye, conjunctivitis is an inflammation of your dog's eye. Symptoms include either clear, watery discharge or greenish-yellow pus-filled discharge, according to WebMD. Your dog also may have very red eyes, he may blink a lot, squint, paw at his face and have trouble keeping his eyes open.

Dry eye — Officially keratoconjunctivitis sicca, or KCS, with this condition your dog's eyes are constantly dry, itchy and irritated because they aren't making enough tears. Instead, their eyes often make a thick, mucus-y discharge. Because infection is a serious risk for dogs with dry eye, your vet may suggest artificial tears and possibly antibiotic drops as treatment, according to the Merck Veterinary Manual.

Foreign body — If just one of your dog's eyes has discharge, there's a good chance there's something in it causing irritation. If that's what you suspect, try using an over-the-counter eye wash to flush your pet's eye, suggests the American Kennel Club Canine Health Foundation. Even if your dog appears perfectly fine afterward and the discharge stops, it's a good idea to follow up with your veterinarian. Foreign bodies can cause corneal scratches in the eye and can lead to infection.

Breed issues — Some breeds are more likely than others to have eye issues. Dogs with naturally bulging eyes, such as pugs and Boston terriers (called exophthalmic breeds) are more likely to have eye discharge because their eyes are more exposed. The exposed eyes can attract foreign objects more easily, and these dogs are more likely to injure their eyes, according to MedicineNet. Other breeds have longer hair or eyelashes that can irritate the eyes and cause drainage. If you know this is an issue for your pet, be alert to any issues.

Eye disease — Thick, persistent discharge may be a sign of a more serious eye disease, such as corneal ulcers or glaucoma. If not treated right away, these illnesses can progress quickly and lead to blindness. “Unlike humans where most glaucoma is slow to progress, dogs develop closed angle or congestive glaucoma and the disease progresses rapidly over days,” veterinarian Robert English, D.V.M. tells the AKC Canine Health Foundation. “But when caught early, therapy can resolve the pain and preserve vision in many dogs.”

Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.