I have to agree with pohnpei397 about our motivations to drink alcohol at celebrations today but I also believe there is a historical and traditional aspect as well.
During the colonization of the Americas drinking alcohol was a necessity. During the colonial period, colonists had to drink alcohol (even children) in order to survive. People did not yet understand how boiling water would lead to purification so many who attempted drinking water became sick. However, when the colonists drank spirits and hard alcohol they noticed they did not become sick. Ultimately, prior to advances in science alcohol had to be consumed by the majority of the population in order to survive. As a result, our culture grew around the stability of alcohol.
There are many articles, papers and books written about this. My favorite book on the topic is The Alcoholic Republic: An American Tradition written by W.J. Rorabaugh. You should definitely give it a read if you are doing a research project or paper!
There are a number of possible reasons why our society associates so many celebrations with alcohol. Let us look at two of them.
First, alcohol is, of course, an intoxicant. As such, it tends to lower people’s inhibitions and enables them to act in a more relaxed way. This can be conducive to celebrations because celebrations are predicated on people being relaxed and able to have fun with one another. In other words, alcohol makes it easier for people to let their hair down and act in ways that are associated with celebrations.
Second, the drinking of alcohol typically acts as a marker that separates serious business from fun activities. Most people think that drinking alcohol (particularly in large quantities) is incompatible with the workaday world. When an event includes the drinking of alcohol, it serves to alert us that the event is something outside the realm of everyday life. In that way, the alcohol serves as a signpost that tells us that things are not “business as usual” and makes us feel more as if we are participating in a special function. In short, the mere presence of alcohol can help to create a special atmosphere that puts us in the right frame of mind for celebration.