This explains so much:

A new brain study suggests an opium-like chemical may drive the urge to gorge on chocolate candy and similar fatty and sweet treats.

Researchers discovered this when they gave rats an artificial boost with a drug that went straight to a brain region called the neostriatum: it caused the animals to eat twice the amount of M&Ms they would otherwise have eaten.

The team also found that when the rats began to eat the chocolate-coated candies, there was a surge in enkephalin, a natural opium-like substance that is produced in the same region of the brain.

via Chocolate Gorging Linked To Opium Chemical In Brain.