When people say "just be yourself" and criticize intentions, they confuse the glass box with the personality inside, and they take their own glass's cleanliness (what makes it transparent) for granted
An example of something that makes personality less visible is social anxiety. Changing that kind of stuff doesn't mean that one is wrongly "forcing it" or being less genuine.
Alternative phrasing of my observation: ... they confuse personality with the box surrounding it, and they take their own box's transparency for granted