While not every case of “red eyes” is the sign of a serious disease in dogs, it carries with it enough of a concern to warrant a trip to the vet to ensure that it’s not.
Glaucoma, which is a disease that causes increased pressure inside the eye, often starts with just redness of the sclera (the white part of the eye). Glaucoma can progress rapidly, and not only is it intensely painful, left untreated loss of vision can occur.
The vast majority of redness in eyes is due to simple conjunctivitis, or inflammation and/or infection of the sack around the eye. Typically, this occurs due to irritation from an allergen in the air, or a contaminant such as smoke. Dogs can also pass bacterial conjunctivitis between them.