I can understand why it's difficult for you to imagine how a person could be unable to stop with just one drink. It's similarly difficult for a non-depressed person to understand why a depressed person is miserable even when things seem to be going well, or for a neurotypical person to understand why an autistic person is freaked out by seemingly minor stimuli.

These are, however, all things that have been extensively studied and documented and affect millions of people. It's not necessary for you to be able to really get inside the head of an alcoholic, but that shouldn't keep you from accepting that this is simply how things are for some people.