Winston Smith is at once terrified and demoralized by the totalitarian society in which he lives. Despite the regime's best efforts at mind control, Winston still manages to retain a capacity for independent thought, which naturally leads him to want to overthrow the Party's tyrannical rule.
The most important source for Winston's true feelings is his diary. Winston doesn't want to keep his subversive thoughts all bottled up inside; he feels an urgent need to express them. But in this totalitarian society, expressing thoughts that contradict Party policy is a serious crime, punishable by death, so Winston has to be extremely careful. He figures that confiding his true thoughts about the regime to a diary is the least dangerous option, certainly much less so than telling someone about them.
Winston's diary is the only place where he can express what he really thinks about the regime. He positively loathes Big Brother; we know this because he writes "DOWN WITH BIG BROTHER" in his diary over and over again. And it's Winston's loathing of Oceania's dictator that leads him to take increasingly bold risks—such as reaching out to O'Brien, someone he believes is an opponent of the regime, but who in actual fact is an Inner Party agent. Winston wrongly thinks that O'Brien also hates Big Brother with a passion. The fact that Winston tries to establish a subversive connection to him demonstrates that his diary entries are not just words; he means to act on his opposition to the regime.
Unfortunately, Winston's brave defiance leads to his downfall. In his capacity as a Party apparatchik—a member of the political elite—O'Brien tortures Winston, not just to punish him, but more importantly to force him to love Big Brother. After his horrifying stint in Room 101, psychologically destroyed, his body broken by torture, Winston finally gives in. As he sits in the Chestnut Tree cafe, crying into his Victory Gin, he gazes up at an enormous portrait of Big Brother. At that moment, Winston, with his brainwashed mind, has triumphed over his subversive self, the self that he expressed so eloquently in his diary. In reality, this triumph, such as it is, is really a victory of the Party over Winston—and that victory is complete. For he now loves Big Brother.