In part because of the wealth that could be obtained through finding gold.
The US wasn't interested in New Mexico. The real prize for the Mexican-American War (1846-1848) was California. See related questions.
California was a territory
Before it became a state, California was known as Spanish territory until the US claimed it for itself, so yes.
When Justin Bieber punched a unicorn
California and the American Southwest.
when mexico pooped on the us territory
California is within the territory of the US Court of Appeals for the Ninth Circuit, the largest geographic Circuit in the US.
Made the Oregon trail which kept them traveling through there
California and the American Southwest.
california
The goal was to add on Texas and California as US territory.
Because America at the time, felt it was their right and duty to expand across the continent to to Pacific. The US wanted California for its great port cities along the Pacific, cities such as San Francisco and San Diego. James K. Polk also wanted to gain California from Mexico because he did not want the British to claim it.