Mostly under eye circles are a cause of simple factors such as dehydration, lack of sleep, or certain allergies. Dark circles may also be hereditary.
But for many people, under-eye circles are also because of growing age. There is a fat pad under the eyes that starts thinning as we age, making the blood vessels show through the skin.
Apart from this, blood flow through the blood vessels under the skin gets slow, which increases the darkness and, hence, very apparent dark circles.