When I first picked up the book I was not ready for it---I hated it. It took about ten years for me to read the book, and by the time I finished my view of biology/evolution was forever changed.
After reading and accepting the ideas the person can't say. -"God has a plan for me" or "I'm on a mission from god" To see life positively one has to interpret things like -"I'm so lucky my ID created me" into -"I'm lucky to be in this small habitual space in the galaxy where the sun shines and I don't have to fear death on a daily basis."
Reviews insist it's depressing but I don't think so. I'm not going to personally go shouting at people to "accept the truth". An atheist version of a Jehovas Witness is quite similarly obnoxious.
So I guess some people don't like to question their assumptions about the world, like danieltillett said.