Smoking once a month turns into once a week, turns into once a day, and soon he becomes permanently stoned, hardly ever leaving his room and doing practically nothing all day. As bad as this is, however, if alcohol replaced marijuana in this situation, the person would probably be dead in a horrifyingly short amount of time. Thankfully, occasional users do not often become addicted to pot. I'd also like to stress that there are plenty of other people who only smoke weed and have not tried harder drugs. It is interesting to note that alcohol is not a big interest for most of these people they drink but not often to get drunk.
Alcoholics are a whole different story.
College is a breeding ground for them. Going to any fraternity will convince the skeptic of this. Most people who go out drinking have relatively high tolerances, requiring them to drink a few beers before feeling any effect. It should be of some concern, then, to find out that having a high tolerance is considered a sign of alcoholism. The big problem is knowing when to stop alcoholics can't stop at just one or two beers.