Many reasons can cause acne but to be exact, it is still not known yet. It maybe internal or external factors. Acne can happened when hormonal changes, bacterial or inflammatory disturbance take place.
When there is a rise or changes in the hormones level, it can cause the sebaceous glands to enlarge and make more sebum. Example: purberty age,menstrual cycle, pregnancy and start or stop birth control pills.
Another factor is Heredity. The tendency to have acne problem can be inherited from the parents. If your friend have absolute, clear porcelain skin, it could be their genes too.
Stress could also be one of the contributing reason to breakout. Hence, having enough sleep and drink more water to lessen the chance of getting acne.
Some people who are on medication can also get acne. Certain drugs, including androgens, lithium, and barbiturates, are known to cause acne.
Unsuitable Topical product can worsen the condition of acne.
Non-Oil based cosmetics may also caused acne. People who are oily skin type should use grease free cosmetic products. Using the wrong cosmetic products or not removing your face make-up will cause your face pores to clog. This will also worsen the acne condition.
Environment Factors. People who are staying in high humid country or country who is facing serious air pollution should take extra care on their face. Especially in hot weather country, you sweat and face become much more oilier. Proper daily cleansing is definitely a MUST to get rid of the dirt and oil on your face.