I'm no expert but I think it's a matter of alcohol depressing the rationality of one's judgement. Similarly, for people with ADD or ADHD, Ritalin is supposed to stimulate the ability to sort out and understand external stimuli (and to control impulses).
a lot of these chemical have both "stimulative" and "depressive" effects. -- Jim Devine / "There can be no real individual freedom in the presence of economic insecurity." -- Chester Bowles