Simple Truth: The Common Good Isn't.

Illustration:
As an attempt to serve the Common Good, the United States implemented the so-called "War on Poverty" back in the 1960's.  This "war" began major wealth transference from the productive to the unproductive and incentivized broken families and the very poverty on which it was claiming to do war.  As a result, we have more generational poverty, more broken families, and a national debt greater than our GDP.

Explanation:
When people look at the economy, they see that it acts in certain ways and it is easy to assume that there is some central thing controlling it.  They look at society and, again, it's easy to assume there is some unified force that drives it.  This is not the case.  What we see as "the economy" or "society" is simply the Brownian motion of people's every day choices.  I decide to purchase something, or not, based on its current price.  Millions of other people across the country making that same choice then translate into higher or lower demand for that item, and thus to possible changes in its price.

What this means is that any centralized action will necessarily upset the environment that allows that Brownian motion to occur.  However well intentioned welfare is, it is undeniable that it pays people not to be productive.  If my choices are flipping burgers for minimum wage, or playing x-box in my parents basement for the same amount, it's hard to make the case for flipping burgers.  And, since this same choice is repeated throughout the economy and throughout society, and many people will choose the option for which the Government is providing an incentive, the net result will be more poverty and more dependence.

Viewed that way, anything done "for the common good" is inherently harmful.

"But," I can hear people saying, "What about Defense?  That's for the common good!"  No, it's not.  We may refer to it that way out of convenience, but in reality Defense is specifically in the National Interest.  That is, there is a single, specific entity (in this case, the United States of America- which happens to be a Federation of States, which are themselves composite entities) which will directly and purposefully benefit.  Such targeted policies, being much more limited in scope, are easier to craft and much less likely to run afoul of the Law of Unintended Consequences.

We cannot aim policies toward "the Common Good," or "the Public Welfare," we must recognize that any policy legislatively enacted will have a discrete set of targets with concrete results.  Another good example is the Healthcare legislation: leaving aside the problems with its Constitutionality (which are myriad) there is the fact that, by trying to be comprehensive and "for the common good" it is instead simply byzantine and confusing, such that even the Government doesn't completely understand what it does.  This is the problem with all Big Government solutions, in the end: by attempting to provide some panacea to some perceived problem, the Government instead simply makes the issue more complicated, and creates more problems than it solves.