I recently answered this question on Quora:

I don't think some of them really know, especially in the United States.

In principle, each side is expressing their political opinion and supporting candidates to represent them in government.

In reality, there are elements in both political parties that have morphed into hyper-ideological zealots who view their opposition as un-American traitors. The opposition is the enemy and they must be stopped at any cost. Some of this is the result of a hyper-partisan, two-party system and those who prey upon the primal fears of voters. Some is because of revisionist history that espouses a very narrow view of what it means to be a true American and patriot.

If you describe yourself as a moderate today many people don't know what you are talking about or think that you are a coward, noncommittal, or just ill informed.

Political anger is now a sport. Political compromise is now a shameful sign of weakness. Is it any wonder that people cheer obstructionist politics?

From: How social media bubbles are driving political anger

Political anger is also big business. On both sides, commentators and bloggers keep their audience by getting them angry and making them fearful.

From Rational Review

There are entire websites and hundreds of blogs and articles dedicated to the topics “how to piss off a liberal” and “how to piss off a conservative.”

✅ 25+ Best Memes About Piss Off a Liberal | Piss Off a Liberal Memes

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So, you have constant attacks and battles as true believers on each side embrace the anger and view absolute defeat of the enemy as the only way to win, motivated by politicians and pundits who are more concerned with power than governance. Stoked by emotion, these zealots always view politics as a win/lose situation and never entertain the idea of political compromise leading to a win/win situation. The general public often follows, basing their political opinions on that of the loudest zealot who wins their attention by appealing to their fears. In our quick-info, 24 hour news, social-media-dominated world we have not taken time to logically reason through the implications of these political policies. Most just pick one side or the other. Most don’t understand the game that is played or know how to sort through the information to determine real issues and reasonable solutions.

I am not saying that politics should never get you emotional. I am not saying that politics should never get you angry. Political disagreement and debate is healthy. Blinding hyper-partisanship that prevents compromise and progress is not.