Clearly, David and Dora's bad luck doesn't all stem directly from mismanagement. Taken together, however, they suggest that David and (especially) Dora are too young and innocent to manage their own household, particularly when it comes to matters of money; they lack the necessary shrewdness, David implies, to avoid being swindled. This is in keeping with the novel's emphasis on the importance of personal responsibility, and the role it plays in financial success. It's not that David and Dora have too little money coming in, but rather that they fail to spend it wisely.