I think that Sinclair does a great job in exploring how unchecked capitalism can lead to a sense of personal destruction and create a system where the people who are supposed to benefit from it actually suffer under it. Jurgis starts out as a strong and happy man, but is withered both emotionally and physically with the hazardous working conditions he is forced to endure. Sinclair is pointed in presenting capitalism and the lack of institutional control as responsible for this. The notion of having "to work harder" precludes any understanding that the system might need to be fundamentally altered, but rather places the burden of substantive failure on the shoulders of the individual. This helps to destroy individual happiness and ruin social interactions. Ona's predicament would prove the latter, as the drive for wealth and the lack of control she has over her capitalism driven existence forces her to become a prostitute. In the end, Sinclair depicts a world where things, profit, and money has more value than human beings, who are seen as a means to an end, as opposed to an end in their own right. When Jurgis carries Ona over the snow to the factory, it is a moment of pure love. Yet, it might also be a moment of sheer horror in demonstrating the magnitude that people have to endure in a capitalist system where profit drives all, and people's predicaments are a distant second.
In The Jungle, capitalism is described as destructive because it is shown as favoring corruption between those in power while it denies social progress and advancement to the working class. American capitalism destroys the social and economic hopes of Jurgis Rudkus's immigrant family. In their move from Lithuania to America, they had hoped to find economic wealth and freedom. Instead, they find themselves almost reduced into slavery, forced to work in humiliating conditions to make ends meet. The literal destruction of the family comes when Jurgis's wife, Ona, dies in childbirth. The famous scene of the slaughtered cattle functions as a metaphor for the workers' conditions.
Capitalism also allows those in power to get richer and to prosper through corruption. The bosses of the meatpacking industry can continue to violate health prescriptions to their economic advantage because they bribe health inspectors (the book caused a detailed enquiry which resulted in the passing of stricter legislation on food processing).
To Jurgis, the appeal of socialism is precisely its call for equality and common good rather than the capitalist emphasis on wealth.