If you want to see how people have been programmed to hate, just look at this thread.
If you are a conservative, you call the liberals all sorts of names and use hateful rhetoric to describe anything you see as changing this country, even if it is for the better.
If you are a liberal, you talk about how the right is hateful and wants to exclude people from this country.
What brought all this about? Several things actually. First up was after the 9/11 attack when Jr. told the world if you aren't with us, you're against us. We started then to divide this country.
Then, when Obama was elected and McConnell and the GOP got together the night after the election to say they wanted to make Obama a one term president. He hadn't even been elected for a day when they said that. At least, Trump got his 100 days to be measured by.
Then, the GOP kept up the divisive rhetoric, and that started to spill over into the general everyday conversation of the public.
Combine that with message boards, tweeting and social media where you can express you opinions without fear of very much retribution, and you have the current situation we are in.
Division, like destruction, is much easier than creation and cooperation.