All US citizens are required by law to enter the US with their US passport. Trying to enter the US without it is a crime. Many other countries require their citizens to use their own passports. Some of the countries that require this are Colombia, Mexico, and Switzerland.

My friend holds US and Canadian citizenship. She tells me that she uses her Canadian passport when she travels in Europe and her US passport while in Latin America because she feels that the US Government has more resources in LA to help her if there's trouble.