Acne is not caused by eating chocolate and French fries, or sleeping on a dirty pillowcase or not washing your face enough. Acne is not your fault.
So what does cause acne? Genetics and hormones are the main culprits here. Because you can't choose your parents, genetics are totally beyond your control. If you have a family history of moderate-to-severe acne, you have the greatest risk of developing it yourself, especially during your teenage years. Beginning in puberty the release of androgen hormones is the primary triggering event. And the hormonal influence continues through adulthood as hormone levels fluctuate, especially in women, often triggering acne in women who may have escaped it in their youth. Further complicating the picture are cofactors or promoters - stress, medications, diet, cosmetics, environment, pollution, and rubbing your skin - which can make your acne worse.
Ultimately, your journey to clear skin depends on treating your skin on a daily basis and doing your best to avoid the cofactors you can control.