At the risk of sounding like I haven't read the preceding 10 pages (which, honestly, I have not), I'd like to state my reasoning for the US sticking it's nose in everyone's business.
I'll try to keep this as simple as possible without pointing fingers at other countries specifically.
The United States of America is the single greatest experiment in liberty ever undertaken by humanity. As part of this, the goal of this nation is to liberate our human brothers and sisters from the ignorance and oppression that dominate the vast majority of the planet.
The US, joined in the last 40 years or so by a number of other Western-style governments, has stood as a pillar of hope and strength for those around the world who believe that they deserve the chance to make it in a vibrant, fair economy.
Therefore we stand at the head of a formation of humanity with a common goal: liberty and justice for all.
Our strength (economic and military) is the cornerstone upon which modern world security rests. And so, whenever these things are threatened (or the resources that are required to maintain our hegemonic position), we MUST respond.
We must respond to maintain the belief that freedom has a champion. We must respond so that those who would seek to take your liberty think twice about attempting it. We must respond or we lose our ability and credibility to respond in the future. We must respond so that small, free nations aren't swallowed by larger powers and so their systems of liberty and justice aren't trampled by systems of oppression and injustice.
The US must respond to global threats against liberty, the rule of law, and free markets. If we don't, who will?
And more importantly, if the US doesn't respond to and eliminate these threats, who will have the strength to do it in 50 years?
The answer is simple, no nation will have the power to make the final stand against oppression if those forces aren't confronted before there is ever a 'final stand'.