Those annoying permanent shadows under your eyes are generally caused by one of three variables:
Facial Structure
The structure of the facial bones can effect the way light is reflected across the face. For example, those with a deep-set of eyes often appear to have accentuated under eye dark circles
Periorbital Hyper-pigmentation
Pigmentation of the skin caused by an increase in the production or distribution of melanin.
Hyper Vascularity
This condition can be caused by thin skin and aging, as well as loss of fat and collagen.