There are two main causes of thirst: 1. Excessive evaporation of water from the body after exercise or when a lot of energy is consumed, or hot weather, dry air and less water will lead to dry mouth and feeling thirsty. 2. Diseases of salivary glands in the mouth. Due to the existence of diseases in the salivary glands, and lead to a reduction in saliva secretion, will also feel dry mouth and other symptoms. 3, endocrine system diseases, like diabetes, the symptoms of diabetes is three more and one less, patients will often feel dry mouth.