The rationality of capitalism is based on the assumption that the market is the superior mechanism for guiding investments to where they are most needed. When it works, it works wonders, but it doesn't always work, as we've seen over the past few years. Instead of the invisible hand directing the economy to fulfill societies needs, we've seen a corrupt system where the rich and powerful plays the system to their advantage, until the giant casino collapsed.
The paradox of investments is that while markets are efficient at allocating funds between alternative projects over the short-term, they often fail to lead financial decision-makers to invest in long-term projects of great public value. Repairing a bridge at risk of collapsing is an obvious social need, but few investors would take it on. Infrastructure is considered boring, and most private investors would rather chase elusive and hyped up targets such as real-estate (before the crash) and Facebook (not crashed yet). Google, and Facebook are providing great value to the modern global society, but it is a systemic failure that capitalism under-invests in providing a safe and sustainable sturcture for human life, nationally and globally. Hence, capitalism must be balanced by the political system. This is not socialism, just common sense. For more about this dilemma, read this article from Truthout: