To understand the contemporary political world, it is useful to know the underlying psychological causes that make people more beholden to certain political ideologies. In general, the left-right dichotomy stems from a disagreement over the fairness of reality. It’s known from research that people on the left have a more external locus of control, while people on the right have a more internal locus of control. What that means is that the left tends to consider luck and fate and powerful overarching systems to be the determining factors in how one performs in life, while the right tends to consider hard work and personal responsibility as the determining factors.
This leads to interesting correlations. People on the left tend to be more prone to depression, but also tend to have a more realistic view of how the world actually works. People on the right tend to believe the just world fallacy, and in so doing they tend to have better mental health and be more successful. Moreover these tendencies are self-reinforcing. Depressed people find it more difficult to motivate themselves and end up doing worse, while people who believe in their self-efficacy tend to work harder and on average end up going further, which leads them to believe they have more control over their destiny. Furthermore, depending on your experiences of success and failure, you’ll tend to become biased one way or the other.
In essence, it becomes a question of truth or happiness. People on the left choose the truth, while people on the right choose happiness. The reality of course is that hard work is essential to success, but that it is not alone sufficient. You also need a fair bit of luck for all the things that are actually outside your control in life. Nevertheless, it is adaptive to delude yourself into thinking you have control, and if you have a string of successful accomplishments, for whatever reason, you’ll tend to think you were fully responsible for them, regardless of the truth.
Now, this explains to an extent why people are left-wing or right-wing, but why then doesn’t one side ultimately dominate? Fundamentally because both end up self-defeating for different reasons. When right-wing views are the majority in a democracy, it tends that parties are elected that will enact right-wing policies that end up stratifying the natural hierarchy such that the majority of people will end up at the bottom of the pyramid, which leads to the majority becoming more left-wing over time. Then, while the left-wing views are predominant, parties will be elected that enact left-wing policies that equalize society and allow more people to be successful. However, those more successful people will fall prey to the psychological tendencies that come from being successful and become more right-wing. This eventually leads to a majority in the democracy being right-wing and the cycle repeats.