Usa always has to be the big brother for the other countries or not any answer is welcome thank you.
It's not written in stone anywhere that it MUST take that roll, but truth is in many situations, others will not act, others don't have the means, may not have the expertise in certain areas, etc...a wide variety of reasons - God has richly blessed America and to whom much is given, much is expected.
The U.S. likes to "step in" when it will ultimately benefit us. We aren't so quick to "step in" when it is just a human rights issue and will not benefit the U.S. in the long run..
We do seem to step in a lot with things. There are times when we don't though.