Where do "Democrat" and "Republican" stereotypes come from?

I hold doors for women and couples, express little sympathy for hardened criminals, attend church regularly, volunteer to lead my youth group in prayer fairly often and generally try to be the perfect gentleman, so many people are surprised to learn that I'm a liberal. Conversely, I was surprised to learn that many of my polite, laid-back, warm-hearted and fun-loving friends who rarely bring up politics are conservatives. I can typically exchange views with right-wing people and not get offended. In fact, it's quite easy to meet in the center with some people when in a friendly environment. So why is it that Democrats are often characterized as spoiled trust fund babies while Republicans are characterized as bigoted, anti-government zealots? Is the media largely to blame for this polarization?