To push for slavery, invade and take away half of the Mexican territory, occupate Mexican soil regularly, and disregard the will of the Mexican people. These are just a few of these actions.
See related questions for further detail.
General distrust of American intentions, even if these look "well intentioned".
Mexico lost half its territory and the US gained access to the Pacific. Both countries lost thousands of men (mostly volunteers) and at least to this date, provided Mexicans with a good reason to distrust any US involvement with Mexico.
Distrust spoils a relationship
It didn't; Mexico was so friendly to the United States that the Mexican government let American colonists settle Texas and other territories, who ironically, would later revolt, fight for their independence and ultimately secede from Mexico.
Mexicans have a distrust of Yankees (Americans).
1853
Annual earnings of US exports into Mexico accounted for US$114.95 billion during 2009 (est.)
There became a wide spread DISTRUST of the government. "Watergate" was directly involved with the war, and "Watergate" added fuel to this distrust.
There are only two of them: the country of Mexico - the original - and New Mexico, a territory acquired by the US from Mexico.
Gorbachev
Any communist nation.
Mexico is south of the US. consequently the US is north of Mexico