The Party stands for everything that is contrary to humanity. Winston is old enough that he is still experiencing remembrances of life prior to Party control. He can imagine his mother and sister and he knows they died. He has recollection of his wife, not that he really loved her. He recognizes that what the Party is doing is wrong, yet is really powerless to do anything about it.
He hates the Party for the apathy created in him, for the dissappearance of his family, and for the recreation of truth. He hates the Party later in the book because it makes it difficult to experience a relationship with Julia. He hates the lack of privacy he has. He hates the Party in the end because they worked hard to break and torture him.
Most of all, the lies of society and manipulation of the people resound to the reader through the character of Winston.