I'm fortunate enough to have escaped from any serious addictions (outside of maybe eating a little too much garbage when I'm feeling a little down), but my mother's side of my family is absolutely packed to the rim with people who have/had powerful drug and alcohol addictions. I also have a close personal friend who has a similar family history and a brother-in-law who comes from a deeply damaged family as a result of alcohol and drug addiction. So, while I can't offer any insight from a first person perspective, I can offer some from a close outside party.
You should be very thankful that you don't suffer from it, nor have had close enough contact to addiction so that a question like you're asking seems reasonable. I urge you to spend a couple hours reading up on it and how it works (at least the parts we know and understand), how it's defined, and how it's treated. If you can find some intervention videos on youtube, it might be worth sitting through.
For my relatives who suffer(ed) from this kind of addiction, it simply becomes all they think about all the time. If they're at work, they're trying to figure out how to get home earlier to start drinking, or they'll try to figure out how to hide the smell of the alcohol behind powerful aftershave. If they're not drinking they're trying to figure out how to drink, and if they are drinking, they're trying to figure out how to stay conscious long enough to get to the next bottle. In my family at least, the addictions are usually co-present with various psychological ailments. The literature on drug/alcohol addiction is jammed full of information on this co-presence.
To an addict, the thing they're addicted to become everything to them. They'll use everybody around them and manipulate them to feed the addiction. If you can't help that purpose, then they'll throw you away and move on to the next supportive person. It becomes more important to them than sex, food, shelter, etc. If you're down to your last $15 and need to decide between food and booze/drugs, to an addict the choice is obvious, you don't have to eat every day.
The best analogy I can give is that addiction of this nature feels a little like slow motion drowning. If you don't get to the surface to get another breath, it's over. A drowning person will climb on top of their own mother to get to that air, and thus it's the same with substance addiction. Everything else becomes secondary.
Bizarrely, it's not always the substances that are addictive by themselves (like morphine), but a powerful psychological predisposition drives many addictions. For an addict who's been able to go clean/sober, giving them a drink or their drug of choice is a little like putting them back in that drowning situation and asking them if they really need that breath or if they can just hold it a little longer. To them it feels like functionally the same situation.
I've watched several friends and relatives die either directly from their addiction, or from some secondary issue caused by their addiction. It's maddening watching it happen. You can even have full-rational conversations with an addict about why it's bad and terrible and they'll agree and provide thoughtful responses about it. But right after that conversation they'll go and pour a drink. The closest example I can think of right now is to watch a few episodes of "Hoarders", ignore the hoarding itself, and watch the conversations with the hoarders. They're completely lucid, but there's something broken in their psyche that causes them to do what they're doing. An uncontrollable compulsion. If you can get through a half dozen episodes of "Hoarders" without going a little crazy, congratulations. I can barely make it through an episode without wanting to reach into the television and grab the people and shake some sense into them so they can see what they're doing.
To your central question, there are programs that attempt to wean addicts off of their addiction. But usually the goal is to get them down to zero. In many cases it's simply because they are so addicted to their substance that going cold turkey would likely kill them. Getting clean, even off of a serious alcohol addiction, can be medically difficult. It can be unbelievably unpleasant, from seizures to hallucinations.
http://www.webmd.com/mental-health/addiction/alcohol-withdra...
Tolerances of people with very severe addictions can also be mind blowing. Blood alcohol levels that would kill most people might be the baseline an addict walks around with all day.
I know from some of my friends and relatives that AA has literally saved their lives. It's put them into a support structure full of people who also have gone through the same thing. It means they both have understanding, but also BS checkers...people who know when they're being manipulated because they were themselves manipulators. If you're struggling with a serious addiction, concern about deism is the absolute bottom of the list of things you have to worry about, but it is often used as an excuse not to join the program. I would also urge you to read up on the AA program, it's a powerful system full of some very deep wisdom. You can attend an "open" AA meeting if you're curious. It might be boring to a non-addict, but it can be the life-changing foundation for an addict. My relatives who are in AA never miss a meeting, and always find where the local AA meetings are whenever they travel. It's "religious" in the sense that following the program religiously has turned their entire lives around.
I also urge you to read up on the criticisms of AA and compare to other programs. Also, learning about AA and AA-like programs (like Narcotics Anonymous) as well as sound-alike but different programs (like Narcanon) can be useful as well. It's likely somebody you know does suffer from addiction, and understanding some of the things about it and how it works might enable you to save somebody's life.