Tearing and red staining can come from several different things. Excessive tearing can be a clogged tear duct as well as a bacterial or red yeast infection. It can be caused by an eyelash that turns under or even from hair around the eye irritating it. As someone has apparently mentioned, it can be caused by allergies to food, grass, home cleaning products, etc.
Make sure that you are keeping the area under the eye dry. You can do this by wiping with a cotton ball once or twice a day. You can also put a drop of Visine on the cotton ball (NOT in the eye) and wipe under the eye with it. I have had more luck with that than with any of the commercial products.
I generally recommend to my clients to talk to their vets about the tear staining as they can make sure that it is not a clogged duct or other medical reason. They can also prescribe medication that will clear up the bacteria or yeast infection generally at less cost than commercial products like Angel Eyes. I have a whole post on it here at my blog.
Hope this helps.