This is what makes addiction so terrifying; just because someone beats it, does not mean they will forever stay sober. Overcoming an addiction is very possible, but there is a high probability of relapsing.
Addiction is a chronic brain disease that slowly appears over a long period of time. Often a substance, or an activity, that a person initially casually enjoys becomes something that they are not able to live without and the addiction appears. This turns into a constant cycle of addiction, treatment, sobriety and then relapsing and starting the cycle all over again.