O'Brien's Message

In the beginning of 1984, Winston believes that he receives a message from O'Brien saying, "I am with you, i know precisely what you are feeling. I know all about your contempt, your hatred, your disgust. But don't worry, I am on your side!" during the Two Minutes of Hate where he felt he had shared feelings of disgust towards the Party with O'Brien. At the end of the book when Winston is in the Ministry of Love, O'Brien knowing everything that Winston thinks and feels comes to fruition. Although it is obvious that O'Brien does not think like Winston as Winston had thought; and that O'Brien does not agree with what Winston believes in. So why does the end of the message end with "But don't worry, I am on your side"?