The most common causes of dark circles are:

Sun damage: can cause the overproduction of melanin.
Genetics: deeper set eyes, certain eye shapes and your brow structure can all create shadowing underneath your eyes.
Pale skin: a lack of melanin in the skin makes capillaries and other blood vessels more apparent.
Aging: your skin becomes thinner making capillaries more visible.
Dehydration: can make your eyes appear sunken and dull creating the appearance of shadows.
Fatigue: capillaries become more apparent through the skin.

Dark circles for some people are not permanent and will disappear based on lifestyle or environmental changes. For instance, as the seasons change dark circles may become less pronounced or disappear as you get a tan. However, for others dark circles will be permanent.

Other than skincare ingredients and products there are some other things you can do to help reduce the appearance of dark circles:

Wearing sunglasses outdoors protects the fragile skin around your eye from sun damage.
Avoid wearing a thick moisturiser in the morning if you wear make-up as this can cause the make-up to sit into the fine lines around your eyes making them look older and worsen the appearance of the dark circles.