Most of us probably learned early in life that forbidden fruit is the root of all evil. Even if we weren't raised in the Judeo-Christian tradition, the story of Adam and Eve is such a cultural motif that it's virtually impossible to avoid a passing familiarity with it. For those who may be rusty, the summary is as follows:
God told Adam and Eve that everything in the Garden of Eden was theirs for the taking, except fruit from the Tree of Knowledge of Good and Evil. They were content for a time, until a serpent convinced Eve to eat from the Tree, and to get Adam to do the same. As punishment for their disobedience, Adam and Eve were evicted from the Garden, and humanity has had to live with hardship ever since.
Setting aside the vilification of women and serpents that also comes out of this story, one clear message is that it's hard to stay away from things that are forbidden. However, in our culture of extremes, we deal with a wide range of behaviors by veering back and forth between overindulging, and completely abstaining. We overspend, then compensate by deciding not to buy anything at all...until we find ourselves splurging again. We overeat, then compensate by dieting, which just causes us to overeat again. We might even conceptualize sobriety from alcohol in this way - we drink too much, and stop by totally abstaining from alcohol, but then one drink easily triggers a full relapse (yes, I realize there are physiological reasons why abstinence may be necessary - I'm just saying that it may also contribute to why "having one drink" is practically impossible for recovering alcoholics, who tend to find themselves binge drinking in spite of their best intentions).
What causes this pattern, the vicious cycle between extremes? Two things: (1) We respond to perceived excesses by overcompensating in the other direction, rather than forgiving ourselves and moving on with a "clean slate." And (2) feeling deprived of something - making it unavailable to ourselves - predisposes us to over-do it when it becomes available again. From the eating disorders literature, we know that restricting food intake is the biggest predictor of binge eating, for both psychological and physiological reasons. We can extrapolate that at least the psychological half of this equation also applies when we try to restrict other kinds of behavior, as well. The bottom line is that, when we perceive something as scarce, we try to go overboard when we encounter it.
The logical solution is also twofold. First, obviously deprivation does not work as a compensation for excess, because it triggers more excess. Moderation really is the key to achieving balance. Give yourself permission to have some junk food once in awhile, or buy something nice for yourself, or whatever it is, so that it doesn't become that "forbidden fruit" you can't resist. Second, forgive yourself if you do happen to over-do it. It's normal to occassionally eat or spend too much, etc. Avoid drawing any conclusions from it about your character and willpower, other than perhaps that you are normal and human, and entitled to the occassional splurge. You may wonder whether you can/should keep forgiving yourself if you over-do it frequently rather than occassionally. To this, I refer you back to the first solution: if you practice allowing yourself "helpings" of the things you desire, you will likely find the frequency of your splurges decreasing with time.
Have you seen this dynamic of "forbidden fruit" in the behavior of people you work with? How do you achieve/encourage moderation?