When it is treated as anything more than a convenient simplification–or a solid starting place for thinking things through–one of the silliest conceits of economics is the idea that people never act against their own interests. Most often, people act against their own interests because cognitive limitations make it hard for them to figure out the right choice, even though strictly speaking, all the information they need to make  an informed ex ante choice is in front of them. But sometimes, people’s psyches are riven by internal divisions. When someone’s soul is embroiled in a hammer-and-tongs civil war, it is natural and appropriate for others to want to weigh in on behalf of one side or another in that struggle. And sometimes, even when one side of an internal psychic division seems firmly in charge, others may want to foment regime change.  

In On Liberty, Chapter IV, “Of the Limits to the Authority of Society over the Individual” paragraphs 3-4, John Stuart Mill lays down rules for such an intervention. In particular, he argues that punishment and strong social stigma should be off limits, but that other efforts to help people improve their lives are a  good thing:

As soon as any part of a person’s conduct affects prejudicially the interests of others, society has jurisdiction over it, and the question whether the general welfare will or will not be promoted by interfering with it, becomes open to discussion. But there is no room for entertaining any such question when a person’s conduct affects the interests of no persons besides himself, or needs not affect them unless they like (all the persons concerned being of full age, and the ordinary amount of understanding). In all such cases there should be perfect freedom, legal and social, to do the action and stand the consequences.  It would be a great misunderstanding of this doctrine to suppose that it is one of selfish indifference, which pretends that human beings have no business with each other’s conduct in life, and that they should not concern themselves about the well-doing or well-being of one another, unless their own interest is involved. Instead of any diminution, there is need of a great increase of disinterested exertion to promote the good of others. But disinterested benevolence can find other instruments to persuade people to their good, than whips and scourges, either of the literal or the metaphorical sort.

One big reason to limit efforts to change what seems like someone else’s self-destructive behavior to advice and preaching rather than punishing or stigmatizing is that one might be wrong. But another reason is that punishing and stigmatizing cause direct harm. For example, the many people in prison for drug use have lives that were already blighted by drugs now blighted by prison as well. Finally, punishing and stigmatizing may often be ineffective because the elements of a riven psyche one wants to encourage may have trouble seeing a punisher or stigmatizer as friendly.