Don't want to get into politics but...

Why is it "I'm a Democrat or I'm a Republican" - Why isn't it just "This is my opinion on this matter"?

I don't understand why being either (or other) is any more of a concern than anywhere else. Do people really sit around and discuss their political concerns that much? I'd rather talk about you or your issues or your golf game or your grandchildren or what fun TV thing we are going to do next.