Drinking alone is, for some reason, one of the stereotypical signs of alcoholism*. No consideration is given to, e.g., the person who finds the taste of Coca-Cola is enhanced by the addition of rum or bourbon. Many people find a little wine with dinner relaxing after a long day.
Drinking alone is only really a problem if it's done because the bottle is a better friend than any of your companions would be, or to hide the amount of liquor you drink, or because you're ashamed of drinking, or because your sole intent is to get drunk, or reasons such as that. It is furtiveness, rather than mere solitude, that needs to be watched for.
*I don't know if it's really a sign of alcoholism, but people think it is. | 
