It's funny--I remember my mom telling me when I was very young that there were three things you didn't talk about in public (sex, politics, and religion).
But she was also the first to break those rules. And unlike you, who are okay with disagreement, disagreement in my mom's world was always personal, and she would decide to dislike people who disagreed with her, including those closest to her.
I like to talk about these sorts of things, because as you said, they are some of the most real parts of who we are. But when discussion (on these or anything, really) starts to feel at all like a debate, I start to shut down and check out. Because that was the sort of thing that led to arguments and yelling and sadness in my house growing up, serious disagreement always scares me a bit.