In George Orwell's novel 1984, why does Julia pick Winston to be her next lover?

In s novel
,choosesto be her next lover for a number of reasons, including the
following:

• When she first saw him, she immediately sensed that he
  was opposed to the Party and was thus a potential lover, since she also opposes the Party. Thus
  she says to Winston:

Im good at spotting
people who dont belong. As soon as I saw you I knew you were against them.
(p. 122, Signet edition)

• She considers sexual
  relations a means of revolting against control by the Party.
• She hates the
  kind of so-called purity and goodness dictated by the Party.
• She actually
  enjoys sexual relations. Raw physical desire is something the Party cant entirely
  control.
• She takes pleasure in outwitting the Party, even though her
  opposition to it is not especially principled or intellectual.
• She
  recognizes that if the Party can control or suppress sexual instincts, those emotional energies
  can instead by made useful to the Party for its own political purposes.
• She
  recognizes that sex threatens the Party in other ways, since it robs of the Party of energies it
  might be able to use in its own interests. Sexual activity makes the people who engage in it
  briefly apathetic toward the Party.
• She uses her sexual attractiveness to
  help encourage Winston in his own hidden revolt against the Party. With Julia to desire, he has
  something to live for. Thus her affair with him helps strengthen him and (slightly) weaken the
  Party.
• She seems to enjoy taking the initiative in planning the details of
  their trysts.  Having the affair with Winston helps increase her own sense of power and
  independence.

Thus Julia chooses Winston for a variety of
reasons, although she might just as easily have chosen anyone else whom she strongly suspected
of disloyalty to the Party.

For evidence to support the points just listed,
see Book II, Chapters 1-3 of the novel.