Fundamentalists are people in a lot of pain who aren't letting themselves feel it.
What a hypothesis. I am really mulling this over.
This was true for me - I was drawn to black and white thinking when I was hitting new adulthood, at a point when I prided myself on not crying much, when I would eat whole rows of Oreos to avoid feeling certain things I just didn't know how to face.
When I think of every friend or acquaintance who I've seen go through a transformation, go through good intensive therapy, become open and vulnerable - they've always (in every case) dropped a fundamentalist approach as they've moved into their new transparency.
People are drawn to fundamentalism because it's a band-aid, it promises protection, seems to provide a way to control your future by ensuring the bad feelings will go away for good because you are doing The Right Thing All The Time - without having to do the other hard, scary thing of facing pre-existing feelings and experiences. (Or being gentle with yourself, cutting yourself slack, instead of obsessing over your sins.)
A big percentage of adult children of alcoholics convert to the more conservative branches of Christianity. Which is fascinating, because alcoholics avoid their emotions by drinking, hence their children both lack a model from which to learn how to deal with emotions, and don't have a safe place to share their own (because a parent who can't deal with their own usually can't deal with their kids', either).
A church that preaches a loving God who is there to heal all your hurts - while also modeling even more strongly that he is expecting you to toe the line of following his precepts in order to win his love just like you couldn't win your distant earthly fathers? If I was an ACOA, I'd say "Yes please!" too.
God is in the wound, God is in the experiencing of the wound. Crying or getting angry can be the most holy of experiences, just like laughing can. That's what actual Christianity is to me. Jesus and I are united in experiencing the suffering of being human. Everyone around me is an extension of Christ, suffering their own crosses, moving towards resurrection if they want it. Christ isn't holier than or separate from them, Christ IS them.
The center of the cross is also the center in yourself where emotions wait to be expressed, released, and received in gentleness (hopefully also by the presence of some loving other) - making room for much, much more light.