Why do people relapse?

What Makes people relapse? If they work hard and go to a rehab, or are sentenced to jail time and do not do a drug for a long period of time why is it they get out and do it again? The answer to this question could be many things. For one drugs are highly addictive, the "wanting" of the drug may never go away. When someone gets back home and back in the real world they may be around the drug and do it simply for that reason. Some people want to quit drugs, some say they do but really do not, some try, some don't. The sad fact is not all people can be saved, even from themselves. Many non drug users try to help users to quit but they cannot all be saved and users often bring non-users down. I am not quite sure what it will take to make this country drug free, but I doubt I will be around to see it.