Because politics is a team sport for far too many people. The people who cling to an "R" or a "D" often fundamentally agree on many issues with the other side.

Because politics is a team sport for far too many people. The people who cling to an "R" or a "D" often fundamentally agree on many issues with the other side. But because its their team they will spout the vitriol they hear from their "leaders" (think, Rush Limbaugh) because they want to win. This is exacerbated by both sides doing a masterful job over the past 50(+/-) years of making Americans absolutely hate one another. They use that wedge to gain power which is all they really want. It all has little to do with actually making the country better.