I think that part of the reason why each son is unable to settle down is reflected in the lack of emotional stability that each one has in their own lives.  Miller seems to place the responsibility for this condition squarely on Willy's shoulders.  Reverting back to the critique that Miller offers about Willy's definition of success, emotional nurturing and the ability to learn how to be a better man in order to serve as a potential husband and spouse are lessons that Willy does not teach to his sons.  This lack of emotional connection causes a type of deficit in each, making their relationship potential in terms of being able to navigate through the emotional challenges of love near impossible.  Biff is challenged by the love- hate relationship that he has with his father.  This makes it really difficult for him to merely settle down and be married to a girl.  At the same time, Happy's womanizing ways demonstrates him to be emotionally unprepared to address and deal with the realities of what it means to be in a relationship.  Given the fact that the boys' own views of marriage would have to be fundamentally challenged from the perspective gained in how Linda and Willy interact with one another makes it clear that their own abilities to settle down are limited, at best.