As I understand it, you are asking, "Why is immigration enforcement controversial in the United States?"
It may help to realize that most people in the United States are descended from people who immigrated in the last five hundred years. This gives a certain amount of pro-immigrant sentiment.
Historically, it has been relatively easy to cross the US/Mexico border. Some number of people came from Mexico and were living in the US with no intention of returning in 1986. President Ronald Reagan and Senator Ted Kennedy made a deal to address immigration. The basic characteristics of the Immigration Reform and Control Act of 1986 were an amnesty for those living in the US since 1982 and increased enforcement on employers.
Since that time, illegal/undocumented immigration has increased. Now, there are an estimated eleven million living in the United States, up from four million in 1986. Increased border enforcement has lead some seasonal farm workers to stay in the US year round.
Crop picking is an example of a job that very few US citizens do. For some crops, it can be automated. Most grains and staples like corn are picked by machines. But some crops need to be picked by hand, and there is a tradition of Mexican workers traveling north for the harvest. It's low paying and hard labor, not exactly the stuff of which dreams are made. It's also temporary work.
There are also some legal forms of immigration by people who are ethnically similar to people who cross the border with Mexico without legal sanction. In particular, Puerto Rico is not part of the US but does have a protected status. Also, Cuban refugees have a history of being admitted after crossing. Added to legal immigrants and those granted amnesty after 1986, this has created a sizable minority who may feel similar to those excluded by laws restricting immigration.
Another issue is that the US has what is called birthright citizenship. Anyone who is born in the US is a citizen regardless of the parents' citizenship status. So it is quite possible that a child (or children) may be citizens and the parents not.
Some crossed into the US as children but have grown up here with no connection to their home country. And they may have siblings who are citizens of the US and nowhere else.
Further complicating things, many of this group of immigrants are more likely to vote for Democrats than Republicans. There are exceptions. For example, Cubans are more Republican than not (e.g. Marco Rubio and Ted Cruz). But in general, more Hispanic immigrants means more Democratic voters in the short term. So it has become a partisan issue.
Given the way that partisan politics works in the United States, once a party has a position, it has to promote it. So there are a large number of stories about how horrible immigration enforcement is. This is even more so since Donald Trump is known for his anti-immigration stances, and Democrats naturally want to discredit his work.