For me, previous experiences set my ceiling to where it is, or its knowing people who have gone through some of the worst consequences of that that make me not want to get even close.
For example, drinking (I'm 23 btw). Some of the parties I go to have more alcohol consumed than a Pub on St. Patrick's day. And there's always that one person who goes too far with their liquor. Everyone knows someone like this; the show up, get smashed within the first hour the first bottle is open, and spend the night throwing up in the bathroom. And people always talk. They talk about how drunk so-and-so got with such disdain in their voice. Being rejected by your peers is an awful feeling, and I avoid "too many" drinks (more than 2) to try and avoid the stigma. I get really self conscious if ever I do drink. I constantly ask myself "am I drunk?" "am I making a fool of myself?" "do I need to ask for a ride home?" "am I being paranoid?" And even if I do get a ride, I'm overcautious. There have been times where my dd would be in worse shape than I was, so I would drive THEM home. No one else would drive me, so in the end I would drive myself there and drive myself home, which meant no drinking.
I wouldn't trust myself with even 1 beer, not unless I waited about five hours after finishing it. I've known too many people who have died, nearly died, or been arrested because of drunk driving. I knew a girl who was arrested for being over the limit after a single glass of champagne. Knowing all that before I was legal to drink left me pretty terrified of relaxing with alcohol. A lot of my friends don't get it unless they know one of the people who died. Its hard to explain without sounding all dramatic. And they think I'm not having fun when I go to parties, which is completely not true. Its just I have a hard time enjoying drinking when so much has happened.
TL;DR I'm super paranoid about drinking because I don't want to be "that wasted girl" and I know people who've died because of drunk driving.