Daily habits: Such as frequent hand washing and bathing using warm water and harsh soap.
Weather conditions: Exposure to cold air indoors and warm air outdoors and constant exposure to air-conditioning can reduce skin moisture. Cold weather also constricts blood vessels under the skin, limiting oxygen supply to the skin and decreasing oil secretion in the skin.
Nature of skin in every area: The nature of skin varies from one area to another. In some areas, skin can be thin. Some do not even have tissue and sebaceous glands under the skin, like legs, elbows and knees, which expose them to even more dryness.
Lips are another area with thin and sensitive skin, deprived of sebaceous glands that would provide moisture and protection from exterior influences. Lips can get dry and cracked, especially in the winter.
Genetic factors: Skin can be genetically dry.
Skin produces less sebum in older individuals. As the skin grow older it becomes drier and requires more moisture.