What are the reasons behind acne?

Plugged follicles: The vast majority of all acne lesions begin as a comedone, which is a small plugged hair follicle. Contrary to popular belief, the cause of these plugs is not dirt or make up, but rather "sticky" skin cells that line the hair follicle--they don't slough off normally. This is why Retin-A and other similar medications are a mainstay of acne treatment.

Just manage it: Acne results from a complex interplay of heredity, hormones, bacteria and stuff we don't understand. Don't try to identify a particular factor in your life. Simply manage it as aggressively as you need to in order to stay clear until it remits on its own. I was able to stop taking medicine when I was 50. Best wishes.