Because the United States believes we should be involved in every conflict, everywhere, all the time. This upside of this is that we have largely been able to call the shots around the world. Being the world's police man, most countries listen to us. The down side of this is becoming more apparent. It is financially unsustainable to be the world's policeman, and it is strategically unwise. We get in the middle of wars and conflicts and pick sides. The other side will hate us and attack us the same they did their enemy before we stuck our noses into it.
What we have is empire abroad. Empires always end, not necessarily because they are beaten, but because they stretch too thin and fall apart (largely due to financial reasons).