This is usually worse in cold/windy conditions or towards the end of the day whilst looking at a screen (most commonly the TV, phone or computer).
Believe it or not a deficiency in the tear film (the protective layer on the front of your eye) is usually the trigger for the eye watering. As your tear film becomes deficient, the nerves on the front window of you eye (the cornea) are not protected as well and so become more sensitive. This leads the eye to believe that a foreign body is in the eye and so the tear (lacrimal) gland will start to produce more tears in response. This is why you then end up with a watery eye that may also burn, or feel sore or gritty.