Winston believes that the stability of the Party depends on having a lot of paralyzingly stupid people around – like Tom Parsons, his neighbor and coworker.
The Party decides everyone needs to love the Party and not anyone else (kind of like a jealous and controlling lover, but with an unnatural proclivity for torture). So the Party eliminated love among family members, actual lovers, friends, and one-night-stands. The Party trains and encourages children to monitor their parents for symptoms of unorthodoxy. (What! She kissed her daughter on the forehead? To the gallows!)
There is no such thing as private possession in Oceania, either. Except for your brain (and given thoughtcrime, even this is up for grabs), you own nothing and the Party owns everything.
Winston writes the diary for the future, for a time when freedom of thought is allowed, when there’s such thing as "reality" and when, perhaps, history can’t be altered to serve political purposes.
Winston realizes that he is being a bad, bad man. Better hide the diary and get back to work already.