Why are many americans pissed off with immigrants, such as mexicans?

I do not understand why so many american hate immigrants from other countrys. They claim that they steal the jobs, er, no they dont, they provide a backbone for you country in lower sector jobs, otherwise harvard graduates would be working in taco-bell. They claim that they arnt good at english, but who said america should be an english speaking country?? after it, many immigrated there from europian countrys that speak diffrent languages when it was discoverd. And lastly, america is a country that tottaly consistes of immigrants, any american living today had his or her roots in a diffrent country, which moved to america a few generations back, (with the exception of native americans), to me it looks tottaly hypocritical