why cellulite occurs

The causes of cellulite are not well understood, but there are several theories that have been put forth as explanations. Among these are:
Diagram of how cellulite forms

Hormonal factors - hormones likely play an important role in cellulite development. Many believe estrogen, insulin, noradrenaline, thyroid hormones, and prolactin are part of the cellulite production process

Genetics - certain genes are required for cellulite development. Genes may predispose an individual to particular characteristics associated with cellulite, such as gender, race, slow metabolism, distribution of fat just underneath the skin, and circulatory insufficiency.

Diet - people who eat too much fat, carbohydrates, or salt and too little fiber are likely to have greater amounts of cellulite.

Lifestyle factors - cellulite may be more prevalent in smokers, those who do not exercise, and those who sit or stand in one position for long periods of time.
Clothing - underwear with tight elastic across the buttocks (limiting blood flow) may contribute to the formation of cellulite.