In this quote, Williams is emphasizing the fundamental social nature of humans. By comparing us to "social monkeys," he highlights that, like many primates, humans thrive on social interactions and connections with others. The mention of boredom, restlessness, frustration, and sadness underscores the emotional and psychological impact of isolation.
Solitary confinement, as he points out, is considered a cruel punishment because it deprives individuals of social interaction, which can lead to severe mental health consequences. This reflects the idea that our well-being is deeply intertwined with our relationships and social environments. In essence, Williams is arguing that our need for companionship and community is intrinsic to our humanity, and denying that can lead to significant suffering.