The middle layer is aqueous or water. The bottom layers, or mucin layer, lines the surface of the eye. It acts as the glue of the tear film, where it prevents the water layer above it from beading up, or clumping together. Dry eye often causes the feeling of eye irritation, characterized as dry, itchy, red, watery eyes. Dry eye can cause tearing because the irritation causes sensation of particles in the eye, so the crying type of tears take over to wash the eye out.
Various conditions cause dry eyes.
The most common culprit is age. The glands and cells that produce the tears simply wear out. Dehydration in the body can cause dry eye. This can occur in young people who consume large amounts of caffeine, or who don't drink enough water. Some diseases can cause dry eye.