Why should I answer my own question?
That's another good question.
Winston feels he has to rebel against authority because he feels confined: his own thoughts, actions, and beliefs are not his to claim. Big Brother--the Party leader--ultimately controls all aspects of human life in this totalitarian world, and Winston realizes the extent of BB's ruthless dictatorship. Winston fantasizes of breaking free from BB's confinement, but fears the wrath of the Thought Police and possible vaporization. The spark to his rebellion resides within his reckless affair with Julia--who will be a contributor to his own demise--as they continue meeting in secret with the risk of being caught. Winston had always dreamed of rebelling: he simply needed a fuel to his fire in order to carry through with it.
I don't think Winston "feels" he has to rebel against authority, rather, Winston knows he must rebel. Much like Neo from "the Matrix" when Winston "wakes up" he knows that there is no going back. This situation can be exemplified by animals. If an animal is born in captivity and escapes there is no way that the animal would go back to his confinement and give up freedom from authority. With these examples a reader may come to understand why Winston can not help but rebel against the authority in his world known as Big Brother.
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