As your brain chemistry changes and your body becomes used to having drugs or alcohol in your system, you may feel the need to have more. Even though you know that you shouldn’t have another, the desire to do so trumps the knowledge that you shouldn’t. This is a key sign that you have an addiction.
Studies have also found that certain people feel more compelled to drink, due to a larger opioid release in the brain after having that first drink. This stronger response was found more frequently in heavy drinkers, and is an indicator that it really is harder for some people to quit drinking because of the increased physiological response that alcohol has on them. If you consciously want to quit drinking but are unable to, even if you know it's what your body needs, it may be a sign that your brain is already overriding other thought patterns to focus on alcohol consumption.
The difference between knowing something and acting on it is more than just a lack of will power. Drug and alcohol use are proven to make it harder to control ones impulses. If you want to cut down or quit drinking but are having a hard time doing so, get help. Talk to your doctor or consult an addiction treatment facility to speak to someone who understands just how hard it is, and can arm you with the right tools to overcome drug or alcohol addiction.