In George Orwell's novel 1984, why does Julia rebel against the Party in the first place? What are her motives?

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:

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. . . 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.