People who have become physically or psychologically dependent to some degree on their drug or drugs of choice often realise that they're drinking or using more than they used to. They may then make some effort to reduce their drug intake.
This could involve cutting down the amount taken each time; only drinking or using drugs on certain days; switching to another drug (e.g. whiskey to beer, alcohol to cannabis, heroin to methadone) etc. Sometimes attempts to cut down may involve a life change such as moving home or changing job.
It's very common for such efforts to end in total failure, much to the utter bewilderment and dismay of the individual concerned. He or she cannot reduce their drug intake.
They're now faced with the fact that their drug abuse is beyond their control. It has taken on a destructive force of its own. He or she now needs help to deal this problem.
No-one knows why some people become dependent on drugs while others don't. It has nothing to do with lack of will-power or moral weakness. A genetic component may be involved; an individual's family and social environment may also play a part.
But it doesn't really matter why - what's more important is what can be done about it.