I've observed several possible explanations:

1. People are taught certain doctrines and will not question those doctrines - ever. If some new information conflicts with those doctrines, then their faith is being attacked.
2. Some are deeply invested in what a certain doctrine allows or prohibits. Think about the sick rationalizations for slavery in the US back in the 1800s supposedly based on the teachings of the Bible. (Sorry, slavery fails the "love your neighbor as yourself" test). To change their thinking means that they have to admit that they were wrong or give up some privilege or perceived position of superiority.
3. They self identify with those beliefs and anything that contradicts that belief is a personal attack. Basic arrogance.

From my perspective, the teachings of Christ were about humility. Admitting that you were/could be/are wrong and being willing to change. That's the whole core of acknowledging your own selfishness (sin), moving to repentance (change) and seeking God's help in that process. Being combative is not compatible with that, in my views.