(will condense this down, as it is a re-run of a re-run)...
Western culture. Specifically, the Protestant, English-based puritan culture upon which this nation was founded. It's pretty unique in the world, and has caused some of our noblest aims--very few nations have shared these "aims" of ours. It's enabled us to do a lot of good.
But the "downside" of this culture is a societal guilt. ALL cultures have done harmful things, abused others, taken advantage of others, and acted aggressively. What's unique about OUR culture is that we feel "guilty" about these things, and have attempted to atone for our many failings.
It's this guilt that makes us feel we "should" take care of poorer nations. It's our "duty", because we're affluent, and can afford to. It's a fine attribute of our culture, and as I said, it's pretty rare, as cultures go.
The problem with this is, most of our good deeds will have to be done without expecting any special gratitude. That's just the way it is. We do good for its own sake, not to be thanked.
Of course, as others (more cynical than I am
) have suggested, there's always good old fashioned greed--- perhaps it's true that at times we DO expect a "payoff". But if we were really totally greedy, there are simpler ways to "get" things than to be "kind". So in conclusion, I think it's a combination of both--yes of course we're always on the lookout for a good business opportunity. But I stll think we have an awful lot of "good intentions" as the result of our culture, that many other nations don't share.
Short, condensed answer to a very complex question.