That's the entire point. That's why the speech ends with "your move, chief".
Sean (Williams' character) is deliberately being confrontational because Will is avoiding making any psychological progress by putting on a fake mask, an avoidance strategy which has been successful with previous psychologists. Sean sees what Will doing and knows that the only way to get Will to stop is to knock the mask off.
In order to get through to Will, Sean has to make Will stop putting up a front, which Will doesn't want to do. So Sean has to go on the attack and break down Will's resistance. He does that by taking a direction that he know will be effeective: Will's own insecurities about his lack of lived experience.