Skin becomes dehydrated for several reasons. Some people are genetically predisposed and naturally dehydrated. This type of dehydration is usually very deep and tends to be harder to treat. Climate is a big contributor to the hydration level of the skin. People in desert climates are usually battling dehydration because there is so little moisture in the air, while those in a more humid environment don't become dehydrated as easily.
Seasonal weather can affect the hydration of the outer skin.
Winter air is usually cold and dry, coupled with indoor heat that zaps moisture from the air causing the skin to become dehydrated. Using a humidifier to combat this dry, artificial heat can really be beneficial to the skin. Flying can do a number on the skin since the air in planes is extremely drying. Sun exposure can also leave the skin dehydrated. If, for instance, you put a bowl of water outside in the hot sun, it won't take long for the water to evaporate.