How O'Brien tortures Winston because Winston insists what?

O'Brien tortures Winston because he insists that reality is objective and that he can maintain his own thoughts and beliefs independent of the Party's ideology. O'Brien seeks to break Winston's spirit and force him to accept the Party's version of truth, demonstrating the extent of the Party's control over individual thought. Through brutal psychological and physical torture, O'Brien aims to reshape Winston's perception, ultimately leading him to betray his own beliefs and embrace the Party's authority.