I think if you've ever talked trash during a game, like really go hard at it, and seen the effect it has on you, you'd understand. It really amps you up. But you have to get over the fear of failure, because if you fail, it's obviously a lot more embarrassing. If you get over the fear of failure, you feel invincible. It's evolutionary biology. "I'm talking and nothing bad's happened to me. I survived! Nothing can hurt me!". It's a rush. It's not just to get in guys heads, that's a rationalization. It's to get into your own head. It seems to change your hormonal makeup, probably secretes adrenalin and testosterone. It makes you confident and you feel scary to other people. I don't think it's literally possible, physiologically, for Sherman to play the way he does without talking.
People hate it because it's not humble. And that's also the reason it's so powerful. If you want the Seahawks to be better than other people you have to let them feel better than other people. All winners do, anything they say contrary is false modesty.