It's used jokingly now, but it started as a serious expression. Same with a lot of expressions.
"Don't judge me" used to actually be a plea not to judge someone too harshly for what they've just said which revealed something about their morals or beliefs; now it's used jokingly in situations which are similar but not nearly as serious.
"You only live once" is the same. It used to mean that you should live life to the fullest, because it's all you've got. Go sky diving, see the Grand Canyon, stuff like that. Now, as has been pointed out, it's basically an excuse for doing stupid shit. Getting drunk and stealing a Ferrari, then seeing if you can outrun the cops. But the situations in which you'd use the expression are again similar; it's just a matter of degree.