Photographing people, for me, is ever such a delicate task. It almost involves getting the camera to your eye and letting the shutter go whilst at the same time sort of NOT doing that. What do I mean? (I wish I knew) I think the task is best tackled by having more than a passing interest in psychology or, at any rate, what people are like. Not just what they look like.
We all know the situation where a conversation with someone is going well: free, un-stressed, easy and then the camera comes into play and, bing, the atmosphere has changed. Sometimes imperceptably but it’s changed.
If one does get the shot, I’ll wager it is partly because you were paying attention to the person involved rather than just juggling light, exposure, viewpoint and moment.