Modern addiction ideology presents the idea that addiction is based on both genetic and environmental factors. Both nature and nurture. I've found this to be true. People that become addicts struggle with both a mental obsession regarding drugs, and a physical compulsion to continue use despite negative consequences. Some people simply don't contend with the obsession and compulsion factors.
For example, I've seen plenty of people hold onto an 8ball of cocaine, or any amount, for some period of time. Not me personally. If I have any cocaine on me, I'm doing it til it's gone. But I could hold onto, and ration opioids a lot better, to some extent. But even then, I'm going to use the opioids a lot more compulsively then the next man who's not an addict of any kind. Different people are predisposed to different substances based on their internal chemistry. Some people just don't contend with the obsession and compulsion factors, and that's the difference between a normal recreational user and an addict.
It's not a matter of willpower. They can't control how their mind works.
It's a choice to use, but being an addict is not a choice.
Im the same way i could never sell coke because if i have coke in the house im gonna do that coke until its gone. However i can ration morphine, benzos or z drugs granted not as easilyu as a non addicted person would.