I know people who will have a couple of drinks, and then stop when they have had "enough". Sometimes they'll even leave half a drink behind unfinished.
I am not like that. My default is to drink more. I can sometimes stop myself if I really want to or need to, but then I try to arrange things so that I don't want to or need to stop.
I also drink when I don't want to. If I have an important work occasion early the next morning, that does not prevent me from drinking.
The two pertinent facts are:
1. When I drink alcohol, I develop a craving for more alcohol
2. I can't prevent myself from taking the first drink when I want to or need to not drink
For many people it takes a series of unfortunate consequences (DUI, jail, mental institution) before they become willing to ask themselves these simple questions. But the questions can be answered honestly without such consequences.