In my opinion, there is a danger in too much immigration, for a variety of reasons. There is competition for resources and jobs. There is a burden on the welfare state. Some groups of immigrants don't assimilate and may be hostile to the culture. Some are criminal.
However, my biggest concern is that mass immigration is used as a tool of the political left to promote the leftist agenda. If we were to have open borders then the country would lose any resemblance to its founding principles of limited government. We would end up with a socialist government. I think that this is the reality, but people don't see this, and many others would welcome it.
This doesn't mean that I am against immigration, nor am I against immigrants. I just don't want to open the floodgates to everyone. I am appalled by those who say that we should have open borders, and I am hearing more calls for this all the time.
The problem with anyone taking a position against mass immigration is that you are immediately labelled a racist. There is also the danger that your goals look similar to racists groups. When browsing the internet for others who are anti mass immigration, it can be difficult to distinguish between those who just want to protect our country, and those who are hate groups. As an example of this, I present the following video: