Basically people are realizing how much money immigrants will contribute and it's a lot of money to say the least, so I'll use myself as an example.
If I were to become a legal resident I would get a better job. That job would mean that I would start paying taxes and since I don't support anyone, the gov will be taking a pretty hefty part of my check. Next I would start buying and spending all that money I made at work on things I need and thereby contribute to the economy. After a while I would eventually be able to get out of my friends house into my own place, which would lead into even MORE spending because I have to furnish my home with a bed, couch, fridge etc... Then after that I would take the next step and buy a motorcycle, maybe a car depending on how things are and if I have a girlfriend. At the same time I would end up sharing the wealth and making sure my dog gets taken care of and take her to the vet for a check up, treat her to something nice and get her a big bone. As I'm doing this I would still be enrolled in school continuing my education and taking more classes that would require me to buy more books and school supplies. As you can see, all of this would happen in the span of a year or two and it will only continue. All this money and I'm only ONE PERSON. Imagine how much more money will be generated if the thousands of students waiting for the DREAM Act would become legal residents ?