Why does happiness seem to be so elusive for so many people?

I think in Oedipus the truth upsets the King's happiness. He probably could have lived happily in ignorance until his death but truth wouldn't allow it. The truth of his heritage, his marriage, and his throne could not be silenced. Ultimately, reality catches up with him and this brings dispair. Life is tough and complete happiness seems rare and Oedipus couldn't escape this.