There many causes of dehydration. As the body loses water and salts quicker than can be replaced, it gradually becomes dehydrated. The number one cause of dehydration is physical activity, in other words exercise. It is the underlying cause of most sports injuries.
Athletes lose a lot of water in action. They are often dehydrated before they feel thirst. Thirst is more or less the body’s “smoke signal” of dehydration, a last resort. It is strongly recommended to drink water during physical activity. About 20 ounces of water per hour of exercise is the minimum recommended dose; and half ounce per pound of body weight on a regular basis (ie on a daily basis, even without exercise).
Other causes of dehydration include:
Various health concerns such as diarrhea, fever, and vomiting,
Poor access to water (especially in infants and the elderly),
Inability to drink water, for example sick babies that cannot suck on bottle, unconscious people, or the severely injured such as certain head injuries or mouth sores, burn victims, and other people with skin diseases and infections that cause water loss via the skin,
Heat exposure, including sun exposure,
Infections that cause frequent urination, and also diabetes.