Brittle nails are often caused by chemicals, detergents, allergens, and frequent polishing and polish removal. Most of the time, they are simply the result of frequent exposure to water.
Though it may seem counterintuitive, frequent dish-washing, hand-washing, or just plain living in a dry environment can dry out the nails. Natural nails typically get drier with age as well, as it is more difficult for the body to retain moisture. Sometimes, however, brittle nails are the result of an iron deficiency or other medical condition.