The reasons why dark circles appear under the eyes are plenty, but are mostly due to it being hereditary, due to the natural ageing process, and due to lack of proper sleep.  

Reasons for getting Dark Under Eye Circles
There are many reasons as to why we get dark circles under our eyes. The most common reasons are heredity, the natural ageing process, and lack of proper sleep.

There are many other reasons attributed to these dark circles. Dark circles could point towards a liver or kidney that is not functioning as desired. Many who suffer from sinus problems could develop dark circles under the eyes. Persons with a history of eczema, hay fever, or asthma often suffer from dark circles. If the dark circles are caused due to some medical condition, you will know it, because the dark circles will be just one out of possibly many symptoms caused.

Dark circles become more pronounced as you keep ageing. The prime culprit almost always are the veins beneath the skin under the eye which is very thin. This skin is very easily damaged and is always the first to show. Ageing and damage caused by the sun make the skin thin and translucent, allowing the veins beneath very prominent. These veins are what make the skin look dark. The skin also looks baggy and puffed because elasticity of the skin keeps reducing with age read rest of the article