What Arrow showed is that group choice (aggregation) is not like individual choice. Suppose that a person is rational and that we observe their choices. After some time we will come to understand their choices in terms of their underlying preferences (assume stability–this is a thought experiment). We will be able to say, “Ah, I see what this person wants. I understand now why they are choosing in the way that they do. If I were them, I would choose in the same way.”

this is why we can’t have nice things: even rational actors lead to absurd group choices.
2022-03-24:

When people make mistakes, they usually try to make better decisions subsequently. To do this, you have to acknowledge that you made a wrong choice. Next, you have to examine the process by which you made the choice, in order to theorize about what would have produced a better outcome. The next time you face a similar decision, you try to correct your decision-making process.

People can experience bad outcomes when they vote. Your preferred candidate or policy could lose. Or your side could win and produce bad results. But chances are, you will not go through an error-correction process. Very rarely will a voter say, “I made a mistake. What went wrong? I need to review how I made my choice, so that I do things differently the next time.”

There are 2 reasons that voters do not engage in error correction. One reason is that 1 person’s vote almost never affects the outcome of an election. It does not pay to invest effort in figuring out what went wrong and trying to correct it. Another reason is that political outcomes are more complex than personal outcomes.

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