Capitalism requires continuous growth as explained here:http://en.wikipedia.org/wiki/Capitalism ... c_elements
Put simply, the profit (the difference between revenues and expenses) is used as capital to increase production or is invested in other businesses for which a return on investment is expected. In the second case, the borrower has to pay for what is lent plus an interest. Usually, what is lent is used to buy means of production to produce or provide service, and receive a profit (part of which is used to pay for the interest). The only exception is a non-profit organization which does not usually dominate a capitalist system.
Thus, money supply increases through profit or interest, and that in turn is employed to increase production further.
This explains why capitalist economies have or want economic growth each year (i.e., an increase in GDP) and why resource use in the global economy continues to increase in the long term, together with money supply.
You can see the same thing on a small level. For example, individuals generally want a promotion or a raise in order to pay for increasing costs or to buy new things, either to replace something old or to follow what is seen through marketing and advertising. Not surprisingly, when they go to work, they are told that production has to increase, that they have to be more efficient, that they have to sell more than previously to make investors happy, to win against other businesses competing against them, and so on.
How is all this possible? Through private property, which essentially began with enclosures during the late Middle Ages and brought about by force. Prior to that, land was used commonly, and people grew food or took care of animals as they needed, with little surplus. But as soon as armed groups enclosed the lands and referred to them as private property, the same people now had to work for the owners and were paid wages. Since the owners wanted to earn from the land, the people now had to grow more than what they needed, with what was produced sold back to them in markets or sold to other kingdoms or regions in exchange for other goods. Thus started the capitalist mode of production explained here:http://en.wikipedia.org/wiki/Capitalist ... production
Thus, private property is only one component of capitalism. The others are the use of money, exchange, profit, labor power, etc.