As part of the older generation, Winston can still recall a time before the Party and Big Brother. Naturally, it is easier to brainwash and control younger generations who have nothing to compare the current system to. In Winston's case, it's harder to keep him in line because he still remembers something better.
Additionally, Winston has character traits that cause him to resist the conformity and intellectual stifling that is part of the Party. Compared with Julia, who is concerned with more selfish pleasures, Winston's resistance to the Party comes from a place of deep, intellectual rumination. He is naturally curious and has a streak of rebelliousness that causes him to test the limits of the Party's power by doing things like writing "down with Big Brother" in his diary.
Naturally, a person like this would bump up against such a restrictive system eventually, no matter the personal danger it would put them in.