According to WebMD, common causes of dark circles under the eyes include seasonal allergies, a cold or a sinus infection. Additional causes include eating a dinner with a high salt content, fatigue, eczema, rubbing the eyes and hyperpigmentation.
The Mayo Clinic also states that in some cases, dark circles under the eyes are hereditary, often running in families. Sun exposure is another possibility, as the body responds by making more melanin - the pigment which gives skin its dark color. Additionally, common effects of aging, such as thinning skin or the loss of fat and collagen, may make the blood vessels underneath the eyes more noticeable. The area underneath the eyes is thin and often appears darker with extra fluid retention.