It does come naturally. Although it can also be learned. Autistic people for instance have to learn to do this. People who aren't autistic can also benefit by learning to do this and improve their accuracy in reading people.
But if you come to false conclusions this could make you paranoid at least in being paranoid about what others think of you. When deliberately reading people it's best to consider your conclusions as tentative and see how things turn out to see if you've been reading people correctly.
Yes it is very natural for all humans to take note of the person's facial expressions and their body posture when they are engaged in a conversation with them. Our posture and facial expression are a part of the means we use to communicate with other people. Watching people's posture and expression is just a part of the communication process.
I personally think that it comes naturally to people to mentally note facial expressions and body postures when taking pictures. I personally think that it comes naturally to people to mentally note facial expressions and body postures because it becomes more of a natural routine. I personally think that it comes naturally to people.
When two or more people are communicating, the body language and non verbal cues are among the most important clues people will get as to what is really going on. We all take these cues in on some level, but we have been trained to ignore them and instead believe the words even if they are lies.
I believe that when we are younger we learn to read our parents facial and body language. Then, it carries on as we get older and becomes more refined. If you think back to when you were a kid, you knew when your parents were mad by their stance or the way their faces looked when you were in trouble. You also knew when they were pleased with what you did by the way they looked or stood. Now, as I am older, I can tell when people do not want to deal with me, when they might be depressed, or even when they might be bothered by something, and I believe it was all learned at a very young age by my parents.
I think it comes naturally to humans to mentally note facial expressions and body postures. I myself feed off of these nonverbal cues when I am in a conversation with someone. I think most people would have to feed off of these cues. You wouldn't keep talking to someone that just frowned and turned away from you mid conversation.