Gold.
Available ports and harbors.
They wanted to convert Native Americans into Christians.
In 1849, approximately 80,000 Americans migrated to California because they wanted to search for gold and hopefully become rich. This was the start of the Gold Rush.
There were close to 100,000 Japanese-Americans living in California in 1940.
Missionaries, priests, padres, and many numbers of Native Americans lived in California missions.
After Shay's rebellion many Americans wanted a stronger national government. Shay's rebellion was an uprising that took place in 1786.
Americans wanted a transcontinental railroad to aid westward expansion. Settlers were looking for land, and the discovery of gold in California also encourage travel.
The discovery of Gold
They wanted to prove their loyalty to the United States. They wanted to show that they were fighting and loyal to try to stop discrimination.
Many white Americans, especially in what is now the southeastern states, wanted the natives removed to the west because they wanted their lands and the resources under them, and because, generally, they did not like or trust the natives.
California currently has the highest population of Americans of Japanese descent.
Americans wanted Spain to grant them the right of deposit.