excerpt from native speaker by chang-rae lee:

I never felt comfortable with the phrase, had a deep trouble with it, all the ways it was said.  You could say it in a celebratory sense. For corroboration. In gratitude. To get a point across, to instill guilt in your lover, to defend yourself. You said it after great deliberation, or when you felt reckless. You said it when you meant it and sometimes when you didn't.