To understand why Julia rebels against the Party, take a look at Part Two, Chapter Two, when she meets with Winston in the woods. Despite being a member of the Junior Anti-Sex League, Julia claims to have slept with many Party members, including those in the Inner Party. For Julia, therefore, her rebellion is driven by a need to express herself sexually and to satisfy her own sexual desires. She does not want to be constricted by the Party's repressive moral code: she wants the freedom to be with whomever she chooses.

In addition, Julia rebels because she wants to have a "good time," as she explains to Winston in the next chapter. She hates the numerous rules imposed on people by the Party because they are designed to prevent a general enjoyment of life. In contrast to Winston's, Julia's rebellion is not based on Party doctrine or the intellectual dimensions of control; she simply wants the freedom to live as she chooses.

In George Orwell’s novel 1984, Julia rebels against the Party for a variety of reasons.  Many of them are explained in the first three chapters of Book II.  Her reasons include the following:

• She feels contempt for the hypocrisy of many Party members, especially members of the Inner Party.
• She hates the sexual puritanism enforced by the Party.
• She values her own independence.
• She enjoys being unconventional
• She doesn’t care if she is considered “corrupt” and in fact takes pride in thinking of herself in that way.
• She enjoys sexual activity and refuses to allow the Party to dictate such intimate behavior.
• She also rejects the mental controls the Party attempts to impose.
• She takes pleasure in pleasure and thus resists efforts to control her pleasures.
• Her opposition to the Party is mainly personal rather than political per se. She dislikes the restraints the Party imposes on her own particular life.
• She realizes that if the Party can control people’s sexual impulses, it can harness such energies and exploit them in its own interests.  The narrator reports her thoughts as follows:

. . . sexual privation induced hysteria, which was desirable because it could be transformed into war fever and leader worship.

Basically, then, Julia rebels against the Party because she has an independent mind, has a strong sexual appetite, has a strong liking for pleasure in general, and chafes under any kind of external, imposed control.