When your eyes become dry and uncomfortable, the tear glands react by producing large amounts of tears, a process known as reflex tearing. Reflex tears contain more water and less mucus and oils than regular tears. Because reflex tears are of poor quality, they don't do a good job alleviating dryness. Even more tears are produced, creating a vicious circle.
Other causes of watery eyes include the following:
If your eyes are watering, a detailed eye examination by your eye care provider can help identify the cause. Treatment will depend on the cause.
Fekrat, Sharon, MD and Jennifer S. Weizer, MD. All About Your Eyes. Duke University Press, 2006.