Fear and Love are powerful tools.
When you initially send help or bomb/invade a country, there is a strong spike in Fear or Love. It eventually evens out, but there is a smaller long-lasting effect.
Nations that have a healthy fear of America will generally sell their goods for cheaper - they're also more responsive to threats.
Nations which love America will buy your goods for more and are more affected by schmoozing.
Cultivating a world where you have a good mix of Countries that fear you and Countries that love you is a great strategy!