The basic problem with capitalism is that it makes every relationship into one that is essentially about money. Capitalism creates a system in which people are constantly trying to exploit one another rather than treating one another as actual human beings.
As an example of this, Marx claims that the bourgeoisie no longer see even their wives as real people. Instead, he argues, a member of the bourgeoisie sees his wife as "a mere instrument of production." This is what capitalism does to people in Marx's view. It stops them from thinking of other people as human beings to be valued. Instead, it makes them think about how to exploit one another.
Thus, capitalism changes human relationships. It creates a system in which people see one another simply as tools to be used for their own personal gain.