What does it mean when people say "Just be yourself" when you are?

[Mona Realm]

The other night I said on twitter I'm in a bit of a sassy mood so I'll log off.(something exciting happened that night) a friend of mine says "haha yes you are but just be yourself." I said "Thanks! I like being myself."

Why do they say be yourself when you already are?? Is it like saying "don't be ashamed of your sassy side be yourself"??
there are occasions where I get sassy or excited. depends on what happened

[Onora]

It is a way of saying that there is no need to pretend to be something that you are not.
e.g. Use your own accent - don't 'talk posh' just to impress people.
If someone upsets you, you don't have to pretend that it doesn't matter.