http://www.seanedwards.com/shellacked-politics/I believe it’s because we subconsciously wrap our identity into our ideas.
When someone says that X is a bad idea, that implies that anyone who believes X is, at best, a stupid person who likes bad ideas, and at worst, a bad person who likes bad ideas.
Humans are fickle creatures. We have a deep need to protect our sense of self. And it is easy to tie your political beliefs to your sense of self-worth.
Your ideas what you believe represent who you are and what you value. They are integrally linked to your sense of self.
How many times have you gotten into a heated argument with a friend or loved one over something really stupid?
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This is a pretty good article about the difficulty of communication , I recommend it.