Why did American want to make California part of the United States?

old was discovered in California in 1848, leading to the gold rush of 1849 (that's why they're called the Forty-Niners, people). California petitioned for statehood in 1849, and was admitted in 1850.

Prior to 1848, California was a territorial possession of no real worth other than its natural harbors (San Francisco and San Diego), acquired from Mexico as part of the settlement of the Mexican-American War in 1846. The population at the time of the cession was perhaps 10,000 people (not counting Native Americans, of which there were probably between 25,000 and 100,000), mostly hardscrabble cattle ranchers. Most local governments still conducted business in Spanish.

The main reason the US wanted California, prior to the discovery of gold in 1848, was to keep it out of the hands of Mexico and of Russia, and as a fulfillment of the Monrovian doctrine of "manifest destiny": one country spanning the whole continent, sea to sea.