Your eyes may water for various reasons, ranging from allergies to yawning to a serious medical condition. Even weather conditions such as wind or looking into the sunlight will cause your yes to water. Despite the reasoning, all watery eyes occur via the same process.
Basically, watery eyes are a sign of a foreign object or other external force causing the eye to respond in a defensive manner. Meaning - tearing eyes are a method of protection for the eye. The moisture in the eye assists in removing foreign objects such as a contact in the wrong place, dirt or pollen. Many times itchy watery eyes occur, in addition to just teary eyes.
How it works
The tear glands - technically known as the lacrimal ducts - begin the process. You can find these glands under the upper eyelids. Each tear duct runs between your eye and your nose.
Environmental factors are some of the most common watery eye causes. These variety of teary eyes is caused by contact with an onion - both via rubbing your eyes or just being near an onion. Other examples include allergens such as pollen and mold. Allergens typically cause both watery eyes and itchy eyes. Smoke is another example of an environmental factor which causes runny eyes.
Conjunctivitis - more commonly known as pink eyes - is a bacteria, viral or allergen related condition which causes an inflammation and infection in the transparent membrane. Besides watery eyes, pink eye triggers symptoms such as gritty eyes, discharge, itchiness or pain and redness.
Nasolacrimal duct blockage
Nasolacrimal duct blockage occurs sometimes without any known reason. In fact, it is difficult to diagnose the origin of a partially blocked duct. However, the partially blocked duct causes an increased tear production due to the tears not channeling through the proper drainage tubes.
Dry eyes are just as the name depicts - a dry eye. Your eyes natural defense mechanism kicks in to counteract the dryness with increased tear production. This results in a watery eye.
Sinusitis, the common cold and even chicken pox are known causes of watery eyes. Other infections -both viral and bacterial - cause the eyes to water uncontrollably. This is your body's defense of fighting ff the infection. This symptom decreases as the infection clears up.