"When he is confronted by the necessity for a decision, even one which may be trivial from a normal standpoint, the obsessive-compulsive person will typically attempt to reach a solution by invoking some rule, principle, or external requirement which might, with some degree of plausibility, provide a "right" answer....If he can find some principle or external requirement which plausibly applies to the situation at hand, the necessity for a decision disappears as such; that is, it becomes transformed into the purely technical problem of applying the correct principle. Thus, if he can remember that it is always sensible to go to the cheapest movie, or "logical" to go to the closest, or good to go to the most educational, the problem resolves to a technical one, simply finding which is the most educational, the closest, or such. In an effort to find such requirements and principles, he will invoke morality, "logic," social custom, and propriety, the rules of "normal" behavior (especially if he is a psychiatric patient), and so on. In short he will try to figure out what he "should" do.

-David Shapiro, Neurotic Styles