I took a Latino Studies class (which
of course makes me an authority

) but here's my opinion:
Most Mexicans crossing the border leave a lot of their family behind, especially if they're illegal. If you want to communicate, you need to retain the language, and language is also a part of cultural identity. There is no "expected immigrant behavior". Part of what America prides itself on is the freedom it gives to its citizens on how they can act, worship, or pretty much how they live their lives (within reason). If all immigrants in the past have acted "as expected", why do we have cultural enclaves like Little Italy and Chinatown and whatnot? I'm not blind, I do see Spanish becoming more prevalent here in SoCal, but so what? I don't see why anyone should be upset if Latinos suddenly become the majority. Telling immigrant groups to act like a true American is beyond rude in my opinion; it tells me that person has no respect for that person's heritage or for the melting pot (oh, I went there) of culture that is the U.S.
However, I think everyone is missing the bigger picture here, which is
why we have such an influx of Latino immigrants. My theory on this is two causes: NAFTA (with CAFTA signed recently) and corruption south of the border. Government corruption always has people fleeing a country, which is understandable. NAFTA, however, is a little trickier. When people think about NAFTA, they think "Oh yay! Free trade!" but don't consider the consequences. With NAFTA, Mexico is able to buy beans and rice at a lower cost than producing those themselves. This creates a major problem for Mexican farmers, who no longer can make money growing food for their own country. Millions have already been forced to seek work elsewhere, and Mexico isn't really known for being urbanized or industrialized.
Most illegal immigrants are illegal because there is an INTENSE bureaucratic process in becoming a citizen here. Both governments intentionally make it difficult to become a citizen by setting up a horrific bureaucratic maze. It also takes a few years to just get a visa. On another point, there is a surprising number of illegals who pay taxes. The IRS probably has the best idea of how many illegals are in this country, because if you don't have a SSN (which would signify that you're a citizen), they can assign you a random number to use in that field.
Illegals also contribute greatly to the economy. Yes, they do take up jobs, but they're usually jobs that other people really don't want to do. California probably has the highest number of illegals out of all the states, yet still remains as one of the largest economies in the world.