"which theory of attraction suggests that long-term relationships are likely to continue when both partners are committed and put a lot into the relationship and when there are few attractive tempting alternatives around? "

An informal structure in a bureaucracy would be something like so-called "office politics". where there's a formal structure set up for the organization, but then an informal subculture as well that has a way of organizing the bureaucracy further. this can be positive because it sets up an added degree of organization, albeit informal, within the bureaucracy.