Causes:
1. Ear canal infection.
When the ear canal is infected by bacteria, viruses, fungi, or parasites, or when the ear is swollen, painful, or itchy, dogs will frequently scratch their ears with their hind legs. Because the facial area has overlapping branches of the auditory and facial nerves, it is very sensitive. Pain deep in the ear canal can extend around the eyes and simultaneously stimulate tear gland secretion. When excess tears cannot drain through the nasolacrimal duct, they overflow from the inner corner of the eye near the nose. Over time, the brown tear stains on the fur become very noticeable.

2. Nasolacrimal duct blockage.
Inflammation and swelling caused by infection of the nasolacrimal duct can lead to total or partial blockage of the duct. As a result, tears cannot drain normally through the nasolacrimal duct and naturally spill out, causing unsightly tear stains.
3.
Entropion.
Entropion, or the inward turning of the eyelid causing eyelashes to grow inward, constantly irritates the cornea. This condition makes dogs and cats feel very uncomfortable and causes them to frequently scratch their eyes with their front paws. They often appear teary-eyed, which also results in severe tear stains.
4.
Lacrimal gland inflammation.
This is primarily caused by an infection of the tear-secreting lacrimal gland, leading to abnormal excessive tear production. The nasolacrimal duct cannot drain this large amount of tears quickly enough, causing tears to overflow and eventually form ugly tear stains.
5. Genetic defects.
Primary excessive tear syndrome is a hereditary disease where tears are often overproduced, leading easily to tear stains.