Emerson has a point. It isn’t easy to grow into the person you are rather than the person someone else – or society – wants and expects you to be. Other ways to put it: To thine own self be true; Be yourself: everyone else is taken. It requires courage, patience, and strength of character.
Being yourself is a Big Idea, but not a complete or sufficient one. It can easily become an excuse for selfish behavior and a disregard for the value of others. If I were to fill out this big idea, I guess it would be along these lines:
Be yourself – a unique gift to the world that is always connected to every other person. And remember to help others be the selves they were meant to be as well.