The causes are diverse and often multifactorial.

Foreign bodies in the eye or small injuries to the cornea, as well as inflammations and allergies, lead to increased tear production. On the one hand, foreign bodies are flushed out and on the other hand, the tear fluid contains enzymes that support healing.

Persistently dry eyes also reflexively produce an excess of tear fluid. When there is a draft of air, the tear film evaporates and the eyes dry out. To counteract the dryness, the eye produces an excessive amount of fluid. Long periods of concentrated work, e.g. at the computer, sometimes cause reduced blinking, which also leads to drying of the eye and is thus in turn reciprocated by the body with a pronounced tear flow.

It is not uncommon for a malpositioned eyelid or a blocked tear duct to be to blame for runny eyes. Particularly in old age, the musculature and the supporting connective tissue weaken. As a result, the outflow of tears is impeded and the tears overflow.