Be Yourself

So once in middle school some friends told me to "Be Yourself".
Apparently, I was a different person at school and away from school.  Ever since then, I always wonder why is that even a saying.  Sad, serious, silly, or studious are descriptions for different people right? They can also be descriptions of the same person.
My teen wondered out loud "How can I be myself?"  She's right.  How would a young person even know what that is--they are themselves.  They can be no other.  They are still trying to figure out who they are, what they like, what is good and what is not.
Sure, I will be myself whether I'm a sad at the moment or a typically sad person.  Label me "sad".
Normally I'm a silly person, but what about the days I am not in the mood--you know what I mean.  However, maybe I feel more comfortable acting silly when I'm nervous.  That is being myself, but it is not appropriate.
When I am myself, which is pretty much always, I can be described differently in different circumstances.  Like my personality or don't like it--I don't care.  Maybe that's where you are supposed to be for others to stop just saying "Be Yourself."