Acne develops because your sebaceous glands are overactive. These glands normally produce small amounts of sebum, which is a thick mixture of oil and skin cells. When you have acne, these glands go into overproduction and the extra sebum can block your follicles and bacteria can move in, particularly the species called Propionibacterium acnes.
Hormones can cause an outbreak of pimples, or increase the number you get. The hormones that are active during puberty also trigger your sebaceous glands to produce more sebum. The hormones with the greatest effect on the oil glands are the androgens, the male hormones. Both men and women have androgens, but men have more.
In women, these hormones can also cause acne during the menstrual cycle, and that's why women often find that acne continues into adulthood. Hormones found in some types of birth control pills can also cause flare-ups of acne.
Eating junk food and chocolate normally has nothing to do with acne. Greasy hair and skin also doesn't cause acne, but they're often a sign of overactive sebaceous glands, which can cause acne. Research suggests that stress may worsen existing acne, but it doesn't cause it.
A tendency to get acne can run in families.