Tears in one eye can be caused by tear duct obstruction, impaction and conjunctivitis. 1. Lacrimal duct obstruction: Lacrimal duct obstruction is one of the common causes of tearing in one eye. When the patient has lacrimal duct obstruction, the tears cannot flow into the nasolacrimal duct through the lacrimal duct normally, which results in tearing in one eye. 2. Inverted eyelashes: If the patient has inverted eyelashes in one eye, it will cause the patient to have tears in one eye because the inverted eyelashes will produce strong irritation to the eye, which will cause the patient to have tears in one eye. 3. Conjunctivitis: If the patient’s eye has conjunctivitis and other ocular inflammatory diseases, due to the inflammation stimulation, the patient may have monocular tearing. It is recommended that patients with frequent tearing and other discomforts consult a doctor to evaluate their condition and follow the doctor’s instructions for treatment.