The US is not a single entity. There are good and kind people in the US and it's government, and there are bad people. The US does a lot of good around the world, but it also does a lot of bad things around the world, as any superpower tends to do. We help feed people because it's the right thing to do. However, if you were one ot eh people getting 'aid', and you were convinced that the US is the very reason you don't have food to begin with (as many of those people do), then you wouldn't be happy and grateful to the US, now would you??
As with everything, there is no easy answer. Keep in mind that a lot of the "humanitarian" things the US does are not really humanitarian at all, but rather serve to further some agenda or have some strategic benefit. Aid often comes with lots of strings attached, not just from the US, but from any country offering 'aid'. Unfortunately many of the good things that the US and it's people do often get overshadowed by the perception of the US as the schoolyard bully (right or wrong).