This is why some people become alcoholics from the first time they drink, because of the genetics they become hooked from the start. Another reason people become alcoholics is the environment they grow up in. If children are exposed to drinking as a activity in which drinkers are careful and moderate they are more likely to become a responsible drinker. If the children grow up seeing heavy alcohol use and abuse they are more likely to use alcohol in that way. Also, if the children are exposed to large amounts of peer pressure from the kids around them they are more likely to drink.
Alcoholism is also a disease that can be acquired over a long period of time.
Alcoholics may start out as social drinkers who are able to control their drinking, but they may lose this control and be carried into the alcoholic class. Some people may drink daily just out of habit on a controlled level. But they may become careless about their habits and the step across the line to alcohol dependence is a short one. Once they are dependent on the alcohol they will build a tolerance to it, this will cause them to drink more and more to get the same effects. Drinking more only makes their dependence worse and eventually they will develop into a full blow alcoholic.