There are many reasons you might have dry skin. Lots of them are obvious, like being genetically predisposed, indoor dry heating, outdoor low humidity and cold air in winter. If you take long, hot showers or even the water itself might have minerals in it that can change the pH of your skin. Even though you think you're completely rinsing off body wash or soap, that might not be the case and this residue could cause dry, itchy skin.
You might have dry skin from diseases like diabetes, thyroid conditions, psoriasis, or eczema. Many medications can cause dehydration of your skin too. Talk to your doctor about medications you take, both orally and topically, to see if this can be the cause of excessive dryness of your skin.
What you drink might cause dehydration, crazy, right? A refreshing glass of iced tea could be causing your body to lose excess moisture even though it feels like you're quenching your thirst. Coffee, tea, soda, and alcohol all have diuretic properties that can dehydrate.
What else? The natural aging process reduces the production of natural oils and acids that keep your skin smooth and plumped. Stress, lack of sleep and anxiety also contribute to dull looking skin. Even the air you breathe can be the culprit, especially if you are in high-pollution areas or where the air is very dry.
So what can you do? Your best bet is to hydrate multiple times a day, both inside and out. Drink water, LOTS of water. This will help rehydrate your body and flush toxins. Use moisturizers with humectants like glycerin, honey, hyaluronic acid or aloe. This will help draw moisture in the air to your skin. Using moisturizer directly after bathing helps to seal in your skin's natural oils and slow down TEWL
(transepidermal water loss), which is a sciency-way to say you're skin is naturally prone to lose moisture. It will happen, but you can help keep it to a minimum by using lotions, creams and other emollients, especially before you go to sleep at night. Think of all that dry heated air sucking moisture from your skin all night long. Creepy, right?