Answer:
Alcohol is largely a genetic disease that is inherited from our ancestors. When people have the necessary genetic makeup (and in some cases, if they simply drink too much for too long, without the genes), alcohol causes changes in their brain that cause them to need alcohol to feel normal.
When the person reaches a point where they can no longer function normally without alcohol in their system, they are addicted -- an alcoholic. As their tolerance for alcohol builds (another sign of addiction) more and more alcohol is required to relieve the discomfort of alcohol deprivation. From there, everything is downhill, since alcohol always affects judgment and makes us drunk, even if we don't show it.
Eventually we reach the point where we can no longer function effectively at all. At that point, there is a chance that our denial may be overcome, and we have a chance at getting into recovery. As long as the booze is working for us, however, there is very little likelihood that we will be willing to change from a known situation to the unknown land of sobriety.