In the lachrymal glands of the eye, tears are formed that moist the eyes and wash them clean from dirt. The tears run down over the cornea and mucous membrane of the eye. When you blink the eye, the tears pass on through the tear ducts down to the lachrymal sac and finally end up in the nose and throat. If there is an imbalance between the tear production and the outflow to the lachrymal sac, lacrimation arises.
The problem is relatively common and with most people it is a problem that passes. When we are emotional and cry, so many tears are formed that the tear ducts cannot cope with the outflow and therefore the tears run down the cheeks. Strong light and wind, smoke or irritating gas (e.g. from onion or chilli) can also lead to so many tears that the eyes run with water. In addition, allergies, inflammation or foreign body in the eye can lead to an increased amount of tears.
Reduced production of tears where you have dry mucous membranes can paradoxically lead to lacrimation. This is caused by irritation of the mucous membrane.
If the outflow of tears is blocked or contracted, lacrimation arises. This can be seen by inflammation of the lachrymal sac or in the case of trauma where the tear ducts are damaged.
When the tears run down the cheeks, the skin is irritated that then becomes red and you constantly have to dry the eyes for water.
Further reading on Frequent signs.