The past and the future do not exist

Unintended consequences. People like to propose grand schemes that will "fix everything", but refuse to accept that there are downsides to that grand scheme.

Infinity. We're simply too dumb to grasp it. Example:

3*(1/3) = 3/3 = 1
3*(1/3) = 3*(0.333...) = 0.999...
0.999... = 1

That "..." means "it continues to the infinite". And yet when you show this reasoning to people, they keep "looking" for the last 9, to claim that 0.999... is not the exact same as 1.

And that applies to all humans. You might counter it rationally, you might train yourself to recognise "it's infinite, so theoretically it'll behave in a certain way", but you don't grasp it. I don't, either.

That why someone behaves a certain way is only important inasmuch as it determines whether they'll keep behaving that way.

Examples:

• Criminals don't need to be punished but rehabilitated; because blame and guilt aren't important; recidivism is.

• Your lover might have all sorts of reasons they love you, and some of those may seem very romantic and some might seem as unromantic as can be. But as long as they will keep living you, that's what's important.