There are several causes for excessive (too much) tearing.

The lacrimal system is composed of glands that make tears and a drainage system that collects them. Excessive tearing (lacrimation) and defective drainage of tears (epiphora) are associated with blurring of vision and constant discomfort caused by tears running down the cheek. In these instances, it is necessary to learn which of these problems is causing the excessive tearing.

Although dry eye syndrome most commonly is characterized by dryness of the cornea and conjunctiva due to inadequate tear production, it also can cause excessive tearing. Dry eye can affect the quality of the tears produced. In the normal eye the tear film is composed of three layers. The first is an oil layer that helps the liquid portion of the film adhere to the eye’s surface. If there is a malfunction in the oil gland, tears will not adhere and simply run out of the eye. Treatment of excessive tearing depends on the underlying cause.
For more information from other health sites, please visit the following webpages:

MedlinePlus, Watery Eyes
http://www.nlm.nih.gov/medlineplus/ency/article/003036.htm
Mayo Clinic, Watery Eyes
http://www.mayoclinic.com/health/watery-eyes/MY01320
For information from the Nationa Eye Institute on dry eye, please visit the following webpage:
Facts About Dry Eye
https://www.nei.nih.gov/health/dryeye/dryeye