What's the psychological principle behind most people rejecting experiences, people, or things that they don't understand? Rather, why are things that are 'different' so readily eschewed or outright condemned/hated? I understand, of course, that new sensory or actions that go beyond our expected patterns cause frustration. But why is this, psychologically speaking?

Is the simple answer that we humans are just combating animalistic impulses still, and that knee-jerk emotion of frustration to the inexplicable doesn't really have to make sense? Is it just a feeling that merely exists, with little reason or purpose? Or, is there, say, an evolutionary reason behind our reptilian brains lashing out in frustration at the unknown?