Why can I not understand people?First and foremost, you will NEVER fully understand all people due to the facts that many people:
A-) Have no bloody idea who they are as people, beyond a somewhat shallow perspective they may have of themselves. If that were not the case, books like the Secret, which is emotional/pseudo-spiritual snake oil for people who do not know what of lame-dumbed-down Buddhism mixed with straight-up narcissism looks like, would not become a North America best seller.
B-) If many do not take the time to figure themselves out, how are you supposed to do that for them?
C-) In many cases society shapes people, more than they may be aware off.
However, it is possible to understand people better. Specifically how to more of less read people while they interact with you.
Through trial, error and experience. I have been running some social groups for about a decade with thousands of people passing through my events and I have to speak to all of them. After a while, you start picking up certain cues, hints, patterns, etc. It is not a perfect science, of course but you do get better at it with enough practice.
How do I do it?
A-) Really listen to people. Not just the words but how they say it. Both tone and choice of words.
B-) Really look at details of how people move, dress and carry themselves.
C-) Be truly empathetic. Actually care about them. I will say most people are almost "hard wired" to be nice and open to good people. Once I know a few details, you use those in a conversation to get to know them more and so on and on.
But as I said, it is not a perfect science and you will certainly meet people that will make you scratch your head.