Winston tells O'Brien in the end of chapter four that he hates Big Brother. O'Brien responds with the truth that Winston must face in order to be released. That hate must transform into love. The love cannot be a fake love. Winston cannot just say that he loves Big Brother because his lies can be detected. Thus, this love must be real and meaningful. This authentic and genuine love needs to not be coerced, although efforts in room 101 will help that occur.
This becomes a central theme in the last several chapters as the book closes with Winston finally allowing himself to love Big Brother. In fact, the final words of the book demonstrate that this change is absolute and complete when Orwell ends with the four words:
He loved Big Brother.