just have a look at the EU and also Germany with some crazies wanting shariah law...this is Germany we are talking about,with their histories and what not

If you provide real social security for anyone in the country and don't limit immigration at all, you attract people who aren't willing or able to work and want to live off social security.

There is no or a very small impact of regulation on the number of exiled people coming in country. However, making more people illegal let bosses exploit them more. Those workers could not sue their boss because of those regulations, and most conservative unions rely unfortunately too much on legal solutions.

So if a country couldn't limit immigrations, it could exploit more people and bybass human right with regulations against exiled people.

Yes, this is only positive for far-right bosses, and awful for others. But guess who decide in a capitalist economy ?

Many people believe that too much immigration causes the destruction of a nation or of an ethnic group by means such as the disintegration of its political and social institutions, of its culture, language, national feelings, religion, and its economic existence.

If immigration leads to more unemployment, then that is an economic problem, especially in the hypothetical case where the social benefits system is getting more and more strained by an influx of unemployed people. But generally, I think that you can expect that the immigrants will soon find employment. Besides that, there's the cultural aspect that @jet@hackertalks.com mentioned. You could also make the point that the country's infrastructure is more and more stressed as the population grows, but that is fixable and potentially counteracted by the labour potential of the immigrants themselves (i.e., qualified immigrant work forces can make a large-scale infrastructure overhaul possible that will lead to greater national capacities and a net benefit for the entire population).

Aside from these things, I would argue that most of the other reasons boil down to xenophobia or racism.

Infrastructure is a large issue. Border towns can become saturated, which will reduce living conditions, and when immigrants move to larger cities, they can often have trouble finding places to live. A lot of this can be because of a communication barrier. Sometimes that is because there are too few to translate, but there can also be educational issues. As much maligned as the US education system is, it is better than some others, and when your culture eschews school for an early start at earning a paycheck, communication in any language becomes a challenge.

Many issues can be overcome, or at least minimized, by compassionate workers, which many that work with immigrants are, but there isn't enough funding to get compassionate people where they are most needed. Supporting increased budgets at the border isn't always about putting guns on the border, it can be about improving the infrastructure that helps get people where they need to be in more efficient ways. I'm starting to ramble, though, and I think I've given a partial answer to your question.