Thin skin. Skin under the eyes is very gentle; it cannot be characterized as elastic and thick. With the age the skin and fatty layer under the eyes becomes thinner, and blood vessels become more appreciable, that creates effect of dark circles. Ultra-violet beams accelerate processes of ageing and dark rounds under the eyes.
Allergic reactions. Dark circles can be result of allergic reactions to the various substances, such as: pollen, dust, wool of the animals etc. Food allergy also promotes occurrence of dark circles under eyes.
Genetic predisposition. At some people vessels are located close to the skin around of eyes from a birth. This physiological feature is transferred by succession.
Puffiness. In result of infringement of an exchange of a liquid in an organism blood vessels are poured by blood and extend. The reasons of puffiness are the following: smoking, surplus salty and sharp in a diet, diseases of cardiovascular system, a thyroid gland, a liver and kidneys, and also reception of some medical preparations.
Lack of dream. Because of a dream the complexion becomes pale, and blood vessels are more appreciable.
You may also add to this list lack of iron of an organism and deaquation.