I suspect you're approaching things too logically. In my experience most people do something then rationalize why they've done that later, if at all. And that's without any perceptive, or memory problems or anything like that, and assumes good intent-- which a lot of people don't have.

I also suspect some people's brains work so differently from my subjective experience that there's effectively no way to understand them.

A very hard lesson for me was "don't try to use logic to understand irrational people."

I now tend to think of people in terms of, "can I predict a response or action by them?" By observation, you can kind of build a set of rules and start hypothesizing about them and update your model when you get more evidence. But you still need to be mindful that different conclusions can be reached with different or conflicting information.

It worked so well on an ex of mine that she thought I was spying on her somehow, because I often had reasoned out things I shouldn't have known. The most terrible example-- that she was cheating on me. I came to this conclusion from a few bits of evidence-- she wasn't really the curious type. Most of the film, music, etc she knew about were from me. And then one day, she started talking in detail about a film I knew she hadn't seen, and she used a word I had never heard her use before.

This lead me to think, "She's having some sort of social interactions that I'm unaware of, and they're probably watching movies together, and if its being kept secret, its probably something bad."

And I was correct.