"If you're happy and you know it clap your hands..."
What if you're happy and you don't know it? Of course, if you don't know you are happy, how can you be happy? You could be in one of those situations that seem uncomfortable at the time, but later you realise how much you enjoyed it.
And what if you're unhappy and you don't know it? You think you're happy. You clap. Then slowly it dawns on you that you were kidding yourself.