A physical dependence on alcohol will occur gradually for most. What happens is drinking alters the balance of the chemicals in your brain. It will alter glutamate, a chemical that excites the nervous system. It affects GABA, which deals with impulsiveness, and alcohol raises the level of dopamine which is the chemical that gives you pleasure when you drink.

Excessive drinking will deplete or increase the levels of some of the chemicals in your brain, which is part of withdrawal when you try to stop. Drinking over time on a regular or fairly regular basis can produce a physical dependence on or addiction to alcohol. Drinking at least a dozen drinks a week for a male or female is thought to increase the chances of alcohol addiction. Just drinking though is not the only factor involved:

Genetic Predisposition:

It is possible that your genetic make-up can contribute to alcohol addiction. The definition of genetics is: The branch of biology that deals with heredity, especially the mechanisms of hereditary transmission and the variation of inherited characteristics among similar or related organisms.

Classical or formal genetics is the study of the transmission of single genes within families and the analysis of more complex types of inheritance. Behavioral genetics is the study of genetic factors in behavior and disease including, say, mental illness.

Genetics can go as far as how many chromosomes a person has. Genetics are inherited and physical or chemical. It is what you were born with.

Family History:

In keeping with the genetics aspect, it is thought (Mayo Clinic) that your risk is higher if one or both of your parents were alcoholics. Anyone who has attended A.A. meetings for any length of time can make this observation on their own.

Age:

If you started drinking at the age of 16 or earlier, you will be at a higher risk for alcohol dependence.

Sex:

It is still believed that men are more prone to alcoholism than women.

Emotional Problems or Disorders:

If you have low self esteem, or you are prone to depression or anxiety, you are at greater risk for alcohol addiction, because people with these disorders may find relief when they use alcohol and can start self-medicating without even realizing it.

More recently, adults with attention-deficit/hyperactivity are thought to be more likely to become dependent on alcohol as well.