Maybe there is something getting into your eye and then cause your watery eye, such as dust, dirt, or eyelash which causes the irritation in your eye. Your body's defense mechanism will work to push them out of your eye, so your eye keep watering.
If it happens all of a sudden, it is probably because there is a small piece of dust in your eye and your body mechanism is trying to push things OUT of your eye. Wash your eyes for several times to see if it will go better.
It may be caused by dryness or allergies. You can get some eyes drop at the pharmacy. If you wear contacts or eye make up, stop wearing them for several days. If it lasts for more than 3 days, you need to see an eye doctor.