How Can So Many Americans Be Opposed To Immigration?

Because they are greedy and don't want to share.
Lots of people fell that the standards of living will change when other cultures impose their beliefs in our society. It is specially frustrating when they don't seem to care or respect how and why we have certain laws and courtesies. When people grow up with mothers that don't speak the language then the kids seem to get away with more and again don't understand the rules and don't care.

They have seen the effects of mass immigration in other countries and the great strain it can put on the economy and resources. Some schools in UK have over 60 different languages spoken in the school which brings down the level of education for the rest of the children. Many are very violent and there are a lot of knifing, murders, rapes etc. Some migrants bring good skills but many are very poorly educated with hardly any English and do not wish to integrate into the British way of life.