Perhaps the main reason for this is lack of sleep, but for some people it is an inherited condition. As you age, the skin becomes thinner in this area and you can see the underlying blood vessels and veins.
Other causes may be a sluggish liver, allergies, sinusitis, or illness.
So what you can you do?
Use a concealer, go to bed a little earlier than normal, try raising your top of your bed (for sinus problems), cold compresses and plenty of fibrous food in your diet. At last resort, lasers can help take care of the pigmentation problem.