George Washington in his Farewell Address famously recommended against foreign entanglements. At the time this made a lot of sense; the US was an infant, growing country, and a large foreign war could ruin it. However, the same advice that works for a growing infant does not apply to a muscular giant, which the modern US clearly is.
Only through foreign entanglements can the US maintain the current global world order, an order that HUGELY benefits the US government and people through prosperity and power. The overarching goal has always been prosperity and growth through trade, and in the past when we were isolationist we relied on other nations to guarantee the safety of international waterways. When these nations inevitably failed in their duties (think Britain in WWII), US trade and prosperity was threatened.
The only way to make sure something is done right is to do it yourself, and so in order to fully guarantee the safety of international trade, the US stepped out of isolation and asserted itself as the hegemon it always had the power to be, but never the will. It designed the world's economic system around itself (Bretton Woods) and created a military force unmatched throughout history (NATO).
This assertion of US power has made the world the most peaceful that is has ever been. http://www.theglobeandmail.com/globe-debate/in-most-places-around-the-globe-violence-has-stopped-making-sense/article1361591/
This peacefulness allows the US to advance the same interests it has always had — prosperity and growth through trade.
If the US retreats back to its borders, then the resulting power vacuum will destroy this peace as well as everything else that the US helped to create, a clearly bad result for both the world and the US.