This is often call crying. You could be crying because you are experiencing a very intense emotion such as being very happy or very sad. Someone could have also told an extremely funny joke that made you cry.
Several things including dry eyes, allergies, conjunctivitis, blepharitis, and foreign objects cause excessive eye tearing. Over the counter products for allergies and dry eye can be used beforehand.
This could be many things but "watery eye" whihc is the unexplained tearing of eyes can be caused by something as simple as an allergy towards dander or even dust mites. YOur best course of action is to take an allergy pill such as zyrtec and it should go away. if it doesnt consult a doctor
