Winston Smith is depicted as a rational, intelligent man who vehemently hates Big Brother and everything the oppressive government represents. He lives in Oceania, a dystopian world superpower where individuality is virtually nonexistent and the government controls nearly every aspect of society.
As a Party member, Winston is under constant surveillance, forced to work long hours, and lives in fear of the Thought Police. Unlike his fellow Party members, he is a secret political dissident who commits thoughtcrime by writing "DOWN WITH BIG BROTHER" numerous times in his private journal and rebels against the Party by carrying on an affair with Julia.
Winston understands that his rebellious personality will eventually lead to his demise but continues to engage in unorthodox behavior in order to retain his humanity. In Oceania's society, the Party forces its members to practice doublethink, believe every announcement emitted from the telescreen, and demonstrate complete adoration and love for Big Brother. Despite the Party's expectations and demands, Winston's humanity, intelligence, and emotions prevent him from brainwashing himself into loving Big Brother.
Essentially, Winston rebels because it is inherent in his nature to strive for a better life and express his individuality. He firmly believes that life was meant to be enjoyed and resents the Party for ruining his experience as a human. He is inherently motivated to search for a better way of living and recognizes the Party as his primary enemy. Winston proceeds to embrace his humanity and express his individuality by committing thoughtcrime, carrying on an affair with Julia, and attempting to join the Brotherhood, an enigmatic underground organization dedicated to overthrowing the Party.