After reading Goldstein's manifesto, Winston understands the "how" but not the "why". What is the "why"?

Winston understands Goldstein's book. He understands how this all happened but he doesn't understand why. What is the reason for the party to keep control in this way? The manifesto has its titles lifted from Party slogans. The readings more or less follow the Party's teachings about loyalty, ignorance, war....Winston understands the history of things now but he still doesn't understand why the Party has created this controlled Hell. Certainly anything is better than the Proles and the Outer Party live.