What type of human behavior will you never understand?
Not caring about anyone other than oneself. Or worse, actively seeking to make life worse for anyone other than oneself/the group one identifies with.
I've caught myself saying it a lot lately. I'll just never understand, for example, why anyone in the US is against a universal-type healthcare system. How can they see people suffering under the current system and think "not my problem, they should have just worked harder to get a job with better healthcare coverage" (as if the solution is that simple for anyone.) I see people suffering and it pains me deeply. I don't care whether I know them, or what kind of person they are, or what their race/religion/sexual orientation/gender/social status/economic status/disability status is. They deserve to receive treatment for their health concerns, and to not go broke in doing so. No one should have to choose between eating and medical care, or housing and medical care (or housing and eating, for that matter.)
Another example would be the current push against trans rights and gender-affirming care in US politics. I'll just never understand it, because I see how much happier people are after starting their transitions and it makes me happy. Why would anyone be against that? There's no reason to try to make their lives worse because you don't understand it. Their identity and expression doesn't affect you, just let them be happy for Christ's sake. Imagine how much better the world could be if we allowed people to find happiness in any of these small ways that they can find it.
I guess tl;dr is that it baffles me how selfish people can be.
Edit: pesky spelling mistake