I think JW say that they are happy because the WT tells them that they are.
Emphasis through repetition ... They hear the words "Jehovah's happy people" so much they accept it as fact just as they do everything else. Even though most of them don't look happy at all. How can you be happy when you never measure up, there's always more to do, you didn't capture that last opportunity to informal witness ... and on and on and on? And living everyday thinking 'probably you may be concealed in the day of Jehovah's anger'. Wondering if you had committed an unforgivable sin. For me that was a painful existence.
Living every day with faith in Jesus and in God's grace ... that makes me happy.