This is the trap that we all fall into, and it brings so much suffering to everyone involved. When someone does something we do not like, we tend to invent negative motives that explain the behavior of the other. We think to ourselves "I know why he did it, he will never say it, but I know what he thinks". Isn't it strange that we do not do this when someone does something that pleases us? How much determination is in those words, how much self righteousness.. That feeling of self righteousness should always be a red flag: it's the ego taking sick pleasure in separating us from others.
What I mean is getting really visible on those days when we feel that even the weather is against us, and we would gladly assign it the thoughts we know it is thinking, if only that would not make us look crazy.
I also realize that this being convinced that the person will never admit to what we know they are thinking is another tell sign that the ego is warping the reality. Because maybe what we are convinced about: that the person will never admit it, is completely true. As the thing that the other person will never admit is quite often (I am not sure how often, but perhaps quite often) fabricated by us, and has nothing to do with that other person, and all to do with us. Then of course, the other person will mostly not admit it - as it is mostly not true. But such believing that it is true, though it is a secret, and can never be made explicit, is a way of making virtually anything true. It's a conspiracy theory on the interpersonal scale. And it escalates when two people start talking about it, and start assigning each other various negative intentions. It becomes a parallel reality, or a number of parallel realities.
We cannot know what are the reasons, thoughts and intentions of others. It took me so much time to understand it. All we see are the derivatives of their thoughts and intentions, and derivatives of derivatives. Most people actually have good intentions, though no idea how to go about applying them. And if they do something nasty it is mostly resulting from the fact that they also made wrong assumptions about others.