It flows from tear glands which located under the upper lids..
When the upper lids stimulated, it produce tears to form a thin film over the cornea and protect them - keep them moist and free of dust..
The tear glands will produce more fluid when eyes are irritated and will produce emotional tears when we're happy or sad..
I found this video explains where tears come from..

Where Tears Come From - The funniest home videos are here
There is a disease called Dry Eye Syndrome..
People who suffer from this disease must help themselves in order to stimulate the production of natural tears..
In this case, they have to use artificial tears every 10 to 15 minutes to their eyes..

There are many causes of Dry Eye Syndrome:
a) Aging process
b) Hot, dry or windy climates
c) Contact lense - it absords the tear film

Eyes are the most valuable parts of our body..
With eyes, we can do things easily as we are able to see and know what is around us..
Afterall, do take a good care of your eyes, friends!

Till then~

Daa~