Because, it's the only one you can really be. It who you are. The person who you've come to be. By birth and by the collections of experiences that your life is.
When you are yourself, you shine. There's no filter between your soul and your surroundings. People appreciate you, and if they don't, they don't deserve you, or your time. There will always be people that appreciate you for exactly the one you are. No more, no less. The rest is noise.
The further away from yourself you are, the harder it is to play life. It's harder because it's not the role you have inside, the one you know inside and out. It's hard because you need to come up with things that feels false and distant from your own values & feelings.
It's about finding environments and settings where you can be the true you. The honest and divine person that you are. It might take a lifetime to find. Like the rest of answers in life. But the sooner you can see who you are, what you need, what you want, the more you can give to the world around you.
And Isn't that what life is about?! Giving. Helping. Shining. Doing what you do best. To to be and do things bigger than your own self. Stretch as far as you can, beyond what you thought was possible. Only when you are your true self you can do that. Go do that.