One of human beings' strongest psychological drives is to protect and maintain their sense of self-identity. When this sense of self-identity is perceived to be attacked, questioned, or ridiculed, the psyche responds with powerful self-defense mechanisms designed to help protect one's internal self-image. This self-defense mechanism can often express itself as defensiveness, fear, or insecurity, or aggression and anger, as an extension.
The problem comes in when one's sense of self-identity is strongly correlated with, and derived from, a group identity which requires or assumes agreement with its purported truth. Any questioning of the dogmatic claims of the group identity (particularly rational and/or factual questioning), is instantly translated by the person as a threat not only to the views of the group identity, but in turn, their own sense of self-identity and worth. This then generally expresses through psychological self-defense mechanisms.
Of course, many peoples' worldview and sense of identity isn't based on a fundamental respect for the value of truth, and thus, engaging in debate or discussion with them is not only an emotionally-charged and pscyhologically-draining experience for them, but also a fruitless exercise for anyone misguided enough to argue with them in the hope of teaching or enlightening them.
