All life is an effort at self-replication. Species that succeed survive and evolve. Those that don't enter the fossil record. All higher forms of life are driven by two elemental instincts, sex and self-preservation. (The former may often overcome the later but that's another issue.)
In human beings, these two drives are experienced as lust – and its “higher” form, love – and the chase after the means of survival – in modern life, money. Capitalism feeds off both of these – indeed combines them – and provides a wide range of products and services in return. (Look at how many combinations of human desire are served by social media such as Facebook and Twitter.) The struggle for the legal tender takes two gross forms in modern capitalist economies: the selling of labor and the reaping of profits by those who control any particular means of production. Capitalism requires both (at least as long as some people are needed to produce and run the machines that increasingly have taken over production).
We all seek more money than we currently have, in part from necessity but as much to provide us with a sense of security. We want to survive today and also tomorrow and the day after. We never know when enough might not be enough so we keep wanting more. Seeking riches is simply the highest manifestation of the drive for self-preservation. No other economic system so deeply satisfies this need than capitalism.
Capitalism is a complete economic system. It directly serves the human hunger for more and more and provides the means to satisfy that hunger. It does this through utilizing the division of labor that arises from the various forms of inequality – those which exist by nature, those that derive from human prejudices and vagaries and those that result from the workings of the capitalist system itself. Put simply, some people work to survive and some reap profit from the work (often including their own).
There seems no real alternative to capitalism because it works directly off of our two most basic drives. Many – most of the world's religions as well as Marx, Luddites, socialists, romantics, nihilists etc – have bemoaned this or looked for alternatives. But there seems no other way to organize an efficient, growing economy than through private ownership and a free market. State control and social ownership have no instinctual basis in human nature and don't work in practice.
The problem arises because capitalism is amoral and by itself produces unfair and unjust outcomes. The belief in some invisible hand that somehow makes it all come out right is an outdated religious notion that flies in the face of our actual experience.
The issue of morality and justice comes into it because humans do not live as monads but as members of society. We cannot survive as individuals outside society. The stability of society, any society, depends on a degree of social cohesion sufficient to keep it from breaking down into an Hobbesian state of nature. Government alone – however much a Leviathan – cannot provide sufficient efficient cohesion. Political dictatorship is incompatible with long-term economic prosperity, it dams the flow of individual freedom that drives the innovation that a capitalist economy requires to avoid stagnation. A widespread experience of the fairness and justice of how the economy functions is the only sound basis for social cohesion in a capitalist society. In other words, capitalism must be tempered sufficiently to provide a common sense of “ownership” or it becomes disruptive.
Capitalism must therefore be regulated for its – and our – own good. Such regulation must somehow represent everyone the system serves. It must therefore be democratic. Only through the functioning of democratic governance can the necessary element of fairness and justice be introduced in a way that serves everyone's efforts at self-preservation. Regulating a capitalist economy – placing limits and requirements on private ownership and market activities – by collective decisions taken through democratic means is the only alternative to capitalism. There seems no other.