What causes bags under the eyes?

Let us count the ways, in descending order of frequency:

1.  Heredity.  That's right. It wasn't that night on the town that makes you look like a raccon in the morning. It's all your parents' and grandparent's fault. Some people are born with excess fatty tissue and liquid around the eyes.
2. Fluid retention. The eyelids are the thinnest and softnest skin in the entire body, four times as thin as 'average' skin. Fluid tends to pool in thin portion of the skin.
3. Ageing. The skin of the face, particularly around the eyes, loosens with agr. Age is more likely to cause bags than mere sleepiness or fatigue.
4. Too many smiles and frowns. They not only can build crow's feet but bags.

A less fascinating explanation for many sightings of bags under the eye was noted by Dr Tom Meek of the American Academy of Dermatology in The New York Times:' The circles are probably caused by shadows cast from overhead lighting.'