From an article geared toward parents of teens:

“Alcohol is a depressant, which means it slows the function of the central nervous system. Alcohol actually blocks some of the messages trying to get to the brain. This alters a person’s perceptions, emotions, movement, vision, and hearing.” 

What are these messages getting blocked? I’ve got some ideas:

“Hey, perhaps you shouldn’t text when you’re three sheets to the wind.”
“Hmm, I bet you shouldn’t call your mom at 3 a.m.”
“You should probably go home instead of having another one.”
“Keep it to first base tonight, will ya?”
“How about you just stick to beer tonight — no shots.”