There are two most common causes of dark circles under the eyes: thinning skin caused by age and genetics. The skin under the eyes is thin and delicate to begin with. As we age, skin and the fat pad under the eyes becomes thinner, causing blood vessels to become noticeable. This gives the appearance of dark circles. As for genetic reason, dark circles under the eyes run in families. They tend to be more noticeable in people with fair skin or deep-set eyes. Of course there are other also common causes of dark circles, from allergies and hayfever, fluid retention, lack of sleep, iron deficiency anemia, and dehydration.