Alcohol affects the pleasure and reward center in the brain, which is the part that motivates people to act for survival, including eating, having sex and socializing.
The reward system in the brain makes us feel happy by releasing a neurotransmitter called dopamine during those experiences. Dopamine causes happiness. The brain remembers which experiences caused happiness and programs itself to desire those experiences.
Alcohol is physically and psychologically addictive because it manipulates the reward system in the brain. It helps us relax and let loose during social situations, making it psychologically addictive. It also causes the brain to release endorphins, another neurotransmitter that causes pleasure.
When alcohol affects the balance of chemicals in the brain, we lose coordination, get excited or feel sleepy. The brain adapts to these changes so it can function normally in the presence of alcohol. This compensation is called tolerance.
Each type of alcohol is equally addictive, but the way someone consumes alcohol can affect their likelihood of becoming addicted. Taking a shot of liquor or chugging a beer is more likely to cause a dopamine rush than slowly sipping a glass of wine. Drinking three glasses of wine with dinner each night is more likely to cause alcohol dependence than having one or two beers at happy hour once a week.