The main reason Winston hates Julia is because she is alluring to him, yet he knows he can never have her.
Julia wears the sash of the Junior Anti-Sex League in a paradoxical way, "wound several times around the waist of her overalls, just tightly enough to bring out the shapeliness of her hips". The message she communicates is cruelly contradictory - she announces unavailability while at the same time being provocative. Winston hates Julia "because she (is) young and pretty and sexless, because he want(s) to go to bed with her and would never do so, because round her sweep supple waist, which seem(s) to ask you to encircle it with your arm, there (is) only the odious scarlet sash, aggressive symbol of chastity".
Winston also hates Julia because she seems dangerous to him. He "dislike(s) nearly all women, and especially the young and pretty ones...it (is) always the women, and above all the young ones, who (are) the most bigoted adherents of the Party...the amateur spies and nosers-out of unorthodoxy". Julia in particular seems even "more dangerous than most". She gives Winston "quick sidelong glance(s)" when she passes him in the corridor at work which "fill him with black terror". Winston is afraid that Julia might be an agent of the Thought Police (Part 1, Chapter 1).