I think the author is on to something. We are far less willing to ascribe psychological motivations to other adults than we are to children. People are happy to say that a child is "shy" or "jealous" or "grumpy because they are hungry."
It's possible that the construction of adult identity means hiding one's psychological motivations from others, and we learn this from the shame of having adults reduce our needs and desires to something other than our own self-narrative.
This is understandable, but maybe we should be learning the opposite lesson. Every human is a psychological being, and it's actually easier to be empathetic when you're aware of your own motivators and look from them in others.