In the past, people believed that acne only appear in teenagers and that they are caused by lack of hygiene. But today we know that there are plenty of reasons for acne to occur, and that they can hit different generations. In case you have acne, it is important to understand what causes them, so you can determine the appropriate treatment. The belief that lack of hygiene causes acne is long behind us. As a matter of fact, you can even do worse in acne treatment if you wash your skin excessively. What causes acne. Acne can appear for many reasons, and among the most common ones are: change in hormone levels, heavy sweating and using certain medications.