It all starts with getting drunk that first time. Then, you get drunk again, and again, ad infinitum. Over time, it takes more and more booze to get you to the same level of drunkenness as before, so you ... drink more and more booze. Boom! Alcoholism.
That college student who binge drinks four or five nights a week? Chances are, even if he continues that behavior well after graduation, he's probably not going to end up as a hopeless drunk living in a van down by the river. That's because heavy drinking is actually more of a symptom of alcoholism than a cause -- while it seems paradoxical, most people who drink a lot are not alcoholics, or even alcohol dependent. And this fact is one of many reasons it's so hard for actual alcoholics to get help.
So if drinking a lot doesn't make someone an alcoholic, what does? Drinking to cope with problems, to manage stressful situations, and to alleviate depression, to name a few. Alcoholism is more of a physiological and psychological dependence on drinking to make it through life than it is a byproduct of the amount of drinking you do. That's why recovering alcoholics quickly learn that "not drinking" is just one part of the battle -- the mechanism that made them dependent is still there, and simply doesn't exist in some drinkers. For instance, their binge-drinking friend who has no problem going without booze for weeks on end. When that guy says, "I'm not an alcoholic, I just like to drink," he's actually telling the truth, where alcoholics are lying to themselves when they say the same thing.