What is Winston speaking of when he says, "I understand HOW: I do not understand WHY" in part 1, chapter 7?

Afterreceives Emmanuel Goldstein's book, The Theory and Practice of
Oligarchical Collectivism, he proceeds to read about the necessity of continuous
warfare to use up valuable resources while simultaneously maintaining low standards of living
and cultivating anof hysteria. In addition to elaborating on the goals of modern warfare,
Winston reads about the principles of Ingsoc in comparison to the similar philosophies of other
world superpowers as well as the importance of propaganda and doublethink. Winston also reads
about why the Party alters history and how Oceania's society is based on reality control and
founded on contradictory principles.

After reading the valuable information
in Goldstein's book, Winston mentions that he understands how, but does not
understand why . Winston is curious about why the Party goes to such great
lengths to oppress humanity, alter history, and completely fabricate reality. He cannot wrap his
mind around the motivation for the Party...