Why did many americans consider the mexican cession to be a success?

The US managed to access the Pacific Ocean, and therefore the country became a country with access to the two most important oceans of the time.

The Mexican cession or territories lost by Mexico are the territories that Mexico was forced to cede to the United States to end the military occupation of the country between 1846 and 1848, as a consequence of the Mexican-American war. This cession was formalized through the Treaty of Guadalupe Hidalgo, signed in the town of the same name on February 2, 1848. The cession of those lands was a condition for the end of the war, when the troops of the United States occupied the city. from Mexico. The United States paid $ 15,000,000 ($ 298,310,309 in 2005) as compensation, which was half of what they had offered with intent to buy it before the war. The land transferred by Mexico is 14.9% of the total area of the territory of the current United States and 119% of the current territory of Mexico.

a countries leader should be motivated by the common good of the country.