The desire to quit can be shaped by the incentives society creates. If using drugs and partying with fellow drug-using friends means continually being committed to three month stints in rehab, it might encourage that party to get out of that lifestyle.
Also, some people may decide to quit once they've been forced to stop doing drugs for a few months, because it results in lessened chemical dependency, and some time for the parts of their brain damaged from the drug-use to recover, which helps increase self-control.
We forcibly prevent children from engaging in self-destructive behaviour, because sometimes they need someone who knows better to control them, until they mature and do know better. Just because someone turns 18, doesn't always mean this stops applying. In the case of someone habitually using hard drugs, I think a strong argument can be made that it still applies because they lack the maturity and self-control to make these decisions for themselves.
Also, some people may decide to quit once they've been forced to stop doing drugs for a few months, because it results in lessened chemical dependency, and some time for the parts of their brain damaged from the drug-use to recover, which helps increase self-control.
We forcibly prevent children from engaging in self-destructive behaviour, because sometimes they need someone who knows better to control them, until they mature and do know better. Just because someone turns 18, doesn't always mean this stops applying. In the case of someone habitually using hard drugs, I think a strong argument can be made that it still applies because they lack the maturity and self-control to make these decisions for themselves.