What joke did the monster tell in the MV "You are not really happy" in Mayday?

The joke told by the monster is actually "you are not really happy." In the emotional background of MV arrangement, if you hear someone tell you, "You are not really happy." Your first thought must be "can't admit it!" Whether you are really happy or not, you don't want others to think so. So you will try your best to smile at others. The more unhappy you are, the harder you smile and the happier you will be. And when you leave the people you want to show him, that is, those who listen to jokes, monsters, people and noisy cities, they don't have to be so happy or laugh so hard. Later, when the monster told the joke to the girl, because the girl was in a vegetative state, she didn't have the idea to show it to others, only the most real feeling, so she wouldn't laugh at that sentence. At the end of the MV, the monster said to the girl, "I am really happy." The girl believed it and smiled happily. As soon as the camera turned, the monster told us with a happy and sad expression, "She is not really happy."