Milia form for a variety of reasons. Some you can fix, others aren’t so easily dealt with.  Milia are often the result of a problem that has affected the skin’s surface.

• Heavy Skin Care Products
  Smothering your skin with heavy skin care products or hair care items is the most common reason milia form. These comedogenic creams and lotions may prevent the sloughing of dead epidermal skin cells. The eyelids are very thin and more likely to experience problems with milia due to cosmetics. 
• Prolonged History Of Sun Damage
  The formation of milia can also be due to cumulative sun exposure. Aging skin forms a thicker epidermis that may make it far more difficult for skin cells to find their way out of the glands. And thicker skin also makes for more road blocks in the pathway to exfoliation.
• Part of chronic skin conditions e.g. porphyria Cutanea Tarda
  Milia can also be associated with certain skin diseases, such as Porphyria Cutanea Tarda which is an unusual disorder. 
• Genetics
  Sometimes we just inherit certain undesirable skin tendencies.