Usually comes from dirty blood, from an overtaxed liver, which is the filter for the blood. Dirty bowel from eating too many dead foods. Sugary syrups (carbonated drinks as in SODA), CHOCOLATE, CANDY, and other foods with wax added or hydrogenated fats, seem to be big culprits.
I had a problem with acne and changed my diet, cleansed my blood with fasting and going off sugar and acne disappeared in about 2 weeks.