When said to a shy person:
"Just relax and be yourself!"
What if someone has not yet found themselves? And their nervous reactions and unleashed nervous behaviors gets in the way of their true self?
Their letting loose their guard could release these habits that just wind up misrepresenting who this person really is, giving a very false impression to the people who do not know them very well.
So their attempt to "be themselves" before they had "found themselves" could actually be less like themselves than when they decided to be shy....
I overcomplicate things, I know.