It could be genetics. It could be allergies. It could be that you are one of those who need more than 8 hours of sleep each night. It could be you are coming down with an illness. It could possibly be poor nutrition resulting in a deficiency of vitamin K, which allows your capillaries to leak out a bit more blood than usual. It could even be the result of a bad habit of rubbing your eyes and thus breaking the capillaries. A deficiency in B12 can also result in dark circles. Smoking can also cause vascular problems that lead to the circles. Problems in digesting protein can also do it.
It could also be something that simply is happening because the skin around your eyes is some of the thinnest, most delicate skin of your entire body. The capillaries are much closer to the surface of the skin there. Many people’s skin is not only thinner around the eyes, but also more translucent. When your capillaries leak (and most capillaries do a bit) the blood that is breaking down creates a dark “bruise” that shows up as those pesky circles.
According to this site, cucumber slices help to lighten the skin, but also soothe and cool the eyes, causing the capillaries to clamp down a bit to prevent heat-loss. Also, the caffeine in tea bags acts as a remedy for dark circles by diminishing puffiness. Caffeine in tea helps to diminish water in the tissues around the eyes (you’ll notice that many early eye circle creams contain caffeine for this very reason). And chamomile tea also helps because it is slightly anti-inflammatory.
This WikiHow gives various suggestions on how to minimize the circles.
You should probably take a look at your family to see if there is a heredity factor, and you might want to be checked by your doctor for anemia, or vitamin deficiencies. Even simpler is to simply add a multivitamin into your daily routine.