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.
Sjogrens syndrome causes dryness in the mucous membranes of the body including the eye. It affects mostly middle aged and older women. If dryness occurs in the morning, it can be caused by incomplete closure of the eyelids during sleep. The tears evaporate. People who are unsuccessful with contact lenses often have dry eye.