How do americans feel about immigration?

I've noticed a lot of Americans have incredibly 'anti-illegals' views, just wondered a) why they're so worried when they have all that space/money b) if they realise that the reason for illegal immigration is mainly because of such a heinous quality of life in their own countries which is only exacerbated by the developed world's continued lack of support in funds etc and c) whether they realise that they are all immigrants themselves? Objective/constructive answers only please! I'm not attacking, just curious!