Why is it that alcoholics develop nerve damage and behavior changes along with cirrhosis?

Alcohol affects. Alcohol, especially when consumed in excess and over a long period of time, adversely affects every organ in the human body except one, the kidneys. If a person has changes in their personality or behavior while intoxicated, that is a big problem already. Such an individual should try and do whatever it takes to refrain from alcohol. Damage from chronic excessive alcohol use is irreversible.
Alcohol is a poison. Kills liver cells, kills brain cells, kills nerve cells. Also causes malnutrition which makes all the above worse.