Culture: Immigrants

In today's world, people are moving to the US from Mexico, El Salvador, Guatemala and many other places in the world.  They typically assimilate quickly. Has this always been the case? Are the immigrants of today assimilating as fast as previous generations?

From 1901 through 1920's there was an estimated 14,531,197 million new immigrants in the United States. Since 1990 through 2014, there has been a total of 520,604,640 million new immigrants in the United States making up 54% of the U.S's population. 

Immigrants come to America to better themselves because they couldn't in their own country, and as soon as they get here, they realize their culture is different from everyone else's so they try to adapt to a new one. 

My questions are: Why does almost everyone immigrate to the United States? Do immigrants come for work or just to live a better life? If immigrants don't find what they are looking for, do they move back home or stay here and risk their lives? 

http://www.census.gov/