If the man who works all day because he has no friends decided to stop this bad habit and started finishing work early, what will happen to him?
He will start to feel bored and lonely whenever he goes home and the result will be either returning back to the old habit of being a workaholic or developing another habit such as television addiction in order to be able to escape from these bad feelings.
As you just saw quitting the bad requires more than just self discipline, will power and persistence. All these good skills can help in different stages but in such a case they will only help the man to stop for a week or two and eventually he will return back to what he was doing.
Some people believe that will power is enough to combat addiction but I am pretty sure you know many iron willed people who fell a prey to certain types of addiction.
The only way to kill addiction is to find the real psychological cause behind it then use your will power to develop and follow a plan that helps you in getting rid of that psychological factor.
In this example the man must use his will power to develop his social skills, to become more confident and to create a social life, only then he will find himself able to quit this bad habit and will find it easy not to ever return to it again.
So in short, people fail to quit various types of addictions because they incorrectly direct their will power towards quitting rather than directing it to solve the root cause of the problem.