You've seen this before. First it's "Yes, I'm happy. I'm single. I'm in a good place." Then a few drinks later, they're crying on your shoulder because they are SO lonely. That can be cathartic between good friends who are venting. On a date, it could lead to deciding whether that person is worth a second date. At a party, it could lead to more drinks, then a morning after full of cotton mouth and regret.
Most people have a set of inhibitions that cause us to think before we speak (or we should). One of alcohol's earliest effects is to loosen up one's inhibitions, which is why it's called a social lubricant. One or two drinks later, the rational face we present to strangers begins to fall away and reveal our emotional side. People become more open, which is what happened when The Cut asked some single folks to talk about love and relationships, before and after cocktails. -via Tastefully Offensive