You can become so skilled at reading people that you stop allowing anyone to surprise your conclusions.

Skill at reading people is genuinely useful and it can harden into a system that never updates. Once you are confident you know what someone is really after, everything they do gets sorted into the existing theory, including the things that contradict it. The accuracy that made you good at this becomes the reason you stop learning anything new about anyone.

Pick a person you feel you have figured out and ask what would change your mind about them. Then watch for it rather than for confirmation. Being occasionally surprised by people is not a failure of perception; it is the sign that you are still looking.