I just read this article and I think it is really interesting. "Am I happy" is not something people really ask themselves when they are caught up in the day-to-day of their lives and because of that, I think it just gets away from us.
Having suffered from depression in the past, I am always doing a mini-check-up on myself and in my experience, the more I am actively doing with my life, the higher my score on the happiness scale. It's when I have too much time on my hands and seemingly nothing to look forward to in the immediate future that I find myself slipping down into a funk.
Everyone is different though, as this article addresses, but there are things that most "happy" people have in common.
Or, we could get into the whole "What really IS a happy person anyway, and are they really THAT happy after all?" but I will skip that for another day.