Why does King David believe that he has the best story in the Bible in the novel, God Knows?

God Knows is David's proof that he has the best story in the Bible. "I've got the poetry and passion, savage violence and the plain raw civilizing grief of human heartbreak," he brags. "Gory deeds? I've got more than enough for every taste. I have suicide, regicide, patricide, homicide, fratricide, infanticide, adultery, incest, hanging, and more decapitations than just Saul's."