Did...did he brainwash himself into
having a happy personality?
No, he's trying to force himself to fit in with the rest of the guild.
If you notice, he's still doing what the others are doing, not what he wants to do. He still is being a doormat, he's just got a big smile on his face. He's trying to please everyone and not telling them that he's actually really badly off (I expect the headache and the ringing in his head was from him overloading his brain from the last fight).