Lack of Sleep and Fatigue:

Partying all night, lack of sleep, fatigue, sinus infections, pregnancy all these may cause paleness of skin. Pale skin makes blood vessels to appear prominently and give an appearance of dark circles.

Structure of Bone

Eyes set deep in skull bones may develop ark shadow under them.

Exposure to Sunlight

Exposure to sun light increases the level of pigmentation and cause tanning. Same thing applies well for the skin under eyes. When skin under eyes gets exposed to sun it tans and appears dark.