I fear its because religion is mans attempt to reach God, and when he feels he has succeeded, he cannot abide anyone elses claim to have done the same. – Jerry B. Jenkins
Of the primary emotions, fear is the one that bears most directly on survival. Children show fear. Adults try not to, maybe because its shameful, or, in some circumstances, dangerous. The fear response is automatic, though, and your body runs through its reflexes whether you want it to or not. – Sebastian Junger