Politics in America

Politics is really nasty stuff. It's everywhere. And as D.C. politics goes, I've always wondered why, generally speaking, we people are either one way or the other? Democrat or Republican, Liberal or Conservative. I'm convinced too that while we all want to be somewhere in the middle, and claim to be sometimes ('I vote on issues, not on parties', 'I'm a libertarian - no one tells me what to do ....', yada, yada, yada ...), we aren't. We either are or we aren't. On or Off. Ones and Zeros.

but why?