As a starting point to your question, I'll give you this piece of information: The current economical crisis that is devastating Europe is a direct consequence of the subprime crisis in the USA some 5 years ago.
That crisis foretold the immediate future of worldwide business and worldwide jobs depending on the strong USA economy for their maintenance. Such was the impact it had on USA economy, which immediately retracted to unheard levels since the Great Depression, that any business in the world depending on USA imports, was gravely affected. Millions have been estimated to have lost their jobs, while hundreds of thousands of business simply either dissapeared or had to face serious and irrecoverable losses. Asia was particularly affected. It is important to remember that this crisis is now called the 2007-2012 Global Economic Crisis. That alone should answer your question.
I shall enter into why exactly the current Euro crisis that is devastating Europe (and that forced me out of my own country and to immigrate to Africa) is a direct consequence of the subprime crisis, if you wish so. For now, I'll leave that statement for you to take at face value, to avoid making this post bigger than it should be.
The simple matter of fact is that, like mentioned before by others (me included) the world economy and world finances are deeply globalized. Weaker economies have little chance to make an impact. In fact they are potential targets for orchestrated attacks on bigger economies (which would lead us back to the Euro crisis, but let's proceed). But stronger economies hold the grave responsibility of maintaining the health of the economical and financial worldwide status. Because their are big, their every decision impacts on millions of jobs and businesses outside their borders.
USA is the biggest of them all. With its strong globalized economy, USA-based business have been dictating not just only employment levels on other countries, but also, indirectly, affecting it by way of its imports (USA is the biggest importer in the world). More, USA financial system as well as worldwide markets are today strongly globalized, with public and private debts of nations and private businesses being held by foreign countries. And while USA is not by far the biggest money lender, its financial health will affect any country of the world, by way of the effect the financial system has today on the economic system (a corruption of values that I'm strongly opposed, but it was in fact USA who systematically created the conditions for financial institutions and not economic ones to be today the prime responsible for economic growth).
The matter of fact is that, an USA economic crisis based on its national debt could plummet the world to an economic dark age. Hyperbole, you think? Remember the subprime world crisis and think of how insignificant that crisis actually is to your economy compared to a national debt crisis. We are talking of a crisis provoked by no money being lent to a government, which is then imcapable of fulfilling its many economic and social obligations. Now multiply the impact it would have on your own country with the rest of the world. USA has yet to experiment a national debt crisis in its history. But can look to Europe's history to learn how devastating that type of crisis is to a country.
USA debt is largely held by the Chinese and the Middle East (most significantly among these, Saudi Arabia). None of these countries are particularly friendly towards USA foreign policies, neither they have ever shared similiar views on politics. China can even be considered an opponent to Western diplomacy, considering its political agenda on the UN that constantly blocks resolutions supported by pro-western diplomacy. For anyone watching on the sidelines -- that includes Europe -- the current financial situation of USA is explosive and deeply worrying. There's a real fear that USA will default sooner or later. When that happens we are in for the first and largest truly wordwide economic crisis humankind has experienced.
Maybe I sound like a prophet of the apocalypse. But don't take my word for it. Read it yourself. Search the answers to your question yourself. None of what I have said is coming from my own head. The information is out there. Just don't look for it on CNN or Fox news. You know who you can trust for information. Your country is full of economic studies, papers, journals and other material written by some of the most well respect economists. In fact USA itself has been the greatest contributor to an understanding of the advantages and perils of the current economic models. Many of these authors have actually predicted the subprime crisis well before it happened. The only reason I don't present you with links or references is because I never bothered to collect them.
The point is that countries in the G7 are today the all-powerful catalysts of the world economy. Whatever agreements go on those meetings of theirs may be voided by an explosive outburst, provoked by some badly received foreign policy or event (or by a hidden agenda). The reason USA has been immune to a default or the markets hysteria is certainly because of the agreements made in those meetings. No one wants a worldwide crisis that will affect everyone. But not for one moment trust mankind not to capable to shoot its own foot.