why is your brain not you?
It just means that the idea of yourself is merely a construct -- what you identify as "yourself" is essentially a program, or a feeling. Like... I don't know who you are. Let's call you Nostromo. You identify yourself as Nostromo.
Nostromo is not in charge of what Nostromo does, because there is no free will. When you do something, when you say something, when you feel a certain way about anything... I'm sure you think, "Well, that's just me. That's Nostromo. I did that." You have a personal feeling with yourself. Nostromo is you, a person responsible for whatever Nostromo does.
But really, Nostromo is just a way to identify you. Really you are just a human creature with a brain that's operating as it wants to operate. "Nostromo" isn't a free person in charge here. Nostromo is the prisoner of Nostromo's brain.
Your brain IS you. It is completely you. But, see, you're not separate from your brain. That, I think, is how most people who don't understand this/don't know this feel. I know I felt it before I knew all this. People think they are MORE than their brain. Especially if you believe in souls, but even if you don't. People think they can control what they do. Control how they are.
The only thing that happens is... you can be trained. You can be trained to be different. To change or whatever. There's no free will in being trained, but it does happen. As I've said before, we're changing all the time. The littlest things can change us. Our brains are constantly being affected by stimuli, changing us and forming us.