Serious question: why do the US interfere with other countries' decisions?

It's always the same: there's a conflict in Africa or Asia and the US Army is always there. They also give their "opinions" on other countries' issues, which don't have anything to do with the US. What's even more: many European countries want the US to be their allies, why? Is the US like a super-mega-country? Do they have to supervise everything, even if America isn't affected?